Does the computer go to sleep if you select Sleep from the black apple menu?

If not, Use Activity Monitor on your Mac - Apple Support to see if any processes are preventing sleep.

Also look over If your Mac doesn't sleep or wake when expected - Apple Support

If resetting the SMC and PRAM did not resolve the issue but it was temporarily resolved after doing an erase and install, the likely source seems to be a third party application that was reinstalled after erasing the hard drive.  Which applications or software did you install when the issue started to occur again?

Turn off the Wi-Fi before sleep.

This should solve your Mac's insomnia problem.

Trash Existing Wi-Fi Preferences in OS X to Start Fresh

1. Create a new folder on your Desktop called ‘wifi prefs backup’ or something obvious
2. Turn off Wi-Fi from the menu item in the upper right corner of OS X
3. Go to the Finder (the smiley face icon in the Dock), and hit Command+Shift+G to bring up the Go To Folder command, choose the following path exactly:

/Library/Preferences/SystemConfiguration/

1. Hit return to go to that folder, then locate and select the following files:

com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist

1. Move all of these files into the folder you created in step 1 on the desktop
2. Delete them once you have made a backup
3. Reboot the Mac
4. Turn on Wi-Fi from the wireless network menu again in the upper right corner of OS X
5. Now test the sleep feature

Just tried Stargajzer's suggestion and it made no difference. I have also tried shutting off all the various "wake" settings.The screen and keyboard go dark when I close the lid, but the MBP is wide awake.

It looks like I'll have to pay the local store a visit. I'll let you know what they say.

FWIW: MacBook Pro, Late 2013

Just so we're on the same page, this procedure is what worked for me. Be certain that you launch the app store and check for any new operating system updates. it will take a minute or two for it to fall asleep once the cover is closed as it closes files and parks the hard disc drive armature. The white LED in front should then begin a continuous cycle of dimming and brightening. I didn't bother to restore the WIFI files that are instructed to back up and I noticed that my list of known WIFI connections were cleared which is no big deal as the next time I visit those locations I'll simply reconnect to them manually. Please reply if this still doesn't solve your problem. Thanks

Power cycle your router (unplug power, wait ten seconds, reapply power) and click on the "Renew DHCP Lease" button within your network settings, advanced, TCP/IP, Click the OK button. Log into your router and check for firmware updates while your at it.

Here is the whole article for those still experiencing technical difficulty with wifi connectivity.

Fixing Wi-Fi Issues in OS X El Capitan

Oct 16, 2015 - 129 Comments

Though Apple has largely resolved the wi-fi issues that persisted in some Macs with prior OS X releases, some users with OS X El Capitan may encounter wireless networking issues after updating to the latest OS X release. Typically the wi-fi problems are in the form of dropping connections or strangely slow speeds, and the good news is they’re usually an easy fix.

 

For most Mac users who are experiencing issues with wi-fi connections in OS X El Capitan, simply ditching old preference files, followed by creating a new network location with custom DNS settings and an MTU change is enough to resolve whatever wi-fi problems they may have had. This is a multi-step process but not particularly difficult. You’re going to be deleting a few system level preference files and creating a new network location. Before getting started, you should start and complete a backup of the Mac with Time Machine. Don’t skip backups.

Trash Existing Wi-Fi Preferences in OS X to Start Fresh

1. Create a new folder on your Desktop called ‘wifi prefs backup’ or something obvious
2. Turn off Wi-Fi from the menu item in the upper right corner of OS X
3. Go to the Finder (the smiley face icon in the Dock), and hit Command+Shift+G to bring up the Go To Folder command, choose the following path exactly:

/Library/Preferences/SystemConfiguration/

1. Hit return to go to that folder, then locate and select the following files:

com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist
NetworkInterfaces.plist
preferences.plist

1. Move all of these files into the folder you created in step 1 on the desktop (you can also delete them if you’re confident and have made a backup)
2. Reboot the Mac
3. Turn on Wi-Fi from the wireless network menu again in the upper right corner of OS X

If your wi-fi works now, great, but for most users, you’re not quite finished yet! Now you need to create a new custom network location.

Create a New Wi-Fi Network Location with Custom DNS

1. Quit any open apps that are using wi-fi or networking (Chrome, Safari, Mail, etc)
2. Go to the  Apple menu and select “System Preferences”
3. Choose the “Network” control panel, then choose Wi-Fi from the list on the left side
4. Click the “Location” menu and select “Edit Locations”, then click the [+] plus button to create a new location, giving the new location an easily identifiable name like “Fixing My WiFi” and click “Done” to add it

1. Next to “Network Name” join the wi-fi network and authenticate with the router password as usual
2. Next, select the “Advanced” button in the lower corner of Network preferences, then go to the “TCP/ IP” tab, choose “Renew DHCP Lease”

1. Next go to the “DNS” tab, and on the left side “DNS Servers” list, click the [+] plus button to add a new DNS server* – I use 8.8.8.8 and 8.8.4.4 for Google DNS but you can choose whatever you want

1. Next, choose the “Hardware” tab, then next to ‘Configure’ choose “Manually”
2. Change “MTU” to “Custom” and set the MTU number to 1453, then click on “OK”

1. Finally, choose the “Apply” button to set your network changes

* If you’re not sure what DNS to use, you can find the fastest DNS servers for your situation with a benchmarking utility. Typically the fastest servers are Google DNS and OpenDNS, but results may vary per region.Now wireless connectivity should be working flawlessly in OS X, and back at full speed. Try things out by navigating around the web, doing a speed test, and just using the internet as usual. The solution outlined above almost always works to resolve wireless networking issues in OS X, particularly if they occur after updating to a new version of system software or a point release.

Additional Wi-Fi Troubleshooting Tips

If you’re still having trouble with wi-fi in OS X 10.11 or later, try the following:

• Reboot the Mac in the Safe Mode, then reboot again (this dumps caches)
• Reboot the Wi-Fi router the Mac connects to
• Update the Wi-Fi router firmware if an update is available
• Join a 2.4 GHz network N network rather than a 5 GHz G network or B network
• Reset the Mac SMC
• Extreme: try to clean install OS X El Capitan
• Extreme: if all else fails, downgrade from OS X EL Capitan to the prior version of OS X on the same Mac with Time Machine

I've been struggling to find a solution to this problem and this has worked for me. Thanks.