It sounds like your Mac will occassionally not wake from sleep. I would use these troubleshooting steps from the article named:
OS X: When your Mac doesn't sleep or wake
The System Management Controller (SMC) is involved in managing power on your computer. Reset the SMC on your Mac if you are having difficulty sleeping or waking your computer.
Nonvolatile RAM (NVRAM)/Parameter RAM (PRAM) stores some power-related settings.Try resetting NVRAM/PRAM if you are having difficulty sleeping or waking your computer.
Some external devices can prevent your computer from sleeping. Try disconnecting items other than your Apple keyboard or mouse from the built-in USB, Thunderbolt and Firewire ports on your Mac. If this resolves the issue, try reconnecting your external devices one at a time until you find the device preventing sleep or wake. Check the documentation that came with the device, or contact the device manufacturer for more information.
Check to see if the issue still occurs when the computer is started in Safe Mode. If this resolves the issue, it could be related to a third party software item such as:
Try creating another user account, or log in as guest. See if your computer sleeps and wakes properly from the additional user account. If it does, your issue may be related to one of the following:
- a login item
- an app open in the original user account
You can disable login items from the Accounts pane of System Preferences. Try opening only one app at a time in the original user account to determine which app is preventing sleep. If you isolate the issue to a particular app or login item, check its documentation, or contact the developer of that software for more information.
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All the best,