Try what Apple suggests here: http://support.apple.com/kb/HT5372 I would restart my devices after making each kind of settings change that they suggest. That's what worked for me. The main issue is not only 'Private Browsing' which will disable iCloud Tabs, but more importantly to make the Safari Browsing Data 'refresh' on all of the devices. You do this by going to Settings>iCloud>Safari>Toggle to 'Off' on ALL your devices one by one (Then make sure that the device doesn't turn the setting back 'On' while you're still there on that page. It did for me a couple of times and I would have to Toggle it back to 'Off'). At this point, go and close the Safari app on all your devices one by one. Then go back to the Home Screen (on iOS devices) on all the devices one by one. Now, do a full restart on each device (on iOS devices (iPhone, iPad and iPod Touch)) this means HOLD DOWN the 'home screen button' AND the 'sleep/wake' button at the same time UNTIL the Apple Logo comes back on the screen, then let go of both buttons simultaneously. (On a Mac: Apple Menu>Restart) Then, Once they are back up and running go to Settings>iCloud>Safari>Toggle to 'On'. After that, open Safari on said devices give it a minute or two and then try accessing iCloud Tabs on the devices and see what happens.
If it's still a no-go try rebooting all the devices again without changing any of the settings this time.
I had to do this process a couple of times before the iCloud Tabs were all present and accounted for.
Be patient. Make sure you're connected to a good wi-fi connection with all the devices on the same Network. That will help the results.