Ubuntu 14.04LTS. Set to suspend when lid closed.
Mostly it works - often I can use it for a week, suspending and resuming with no problems. But sometimes something changes and it stops working: shutting the lid will do nothing - the machine keeps running.
If the fan comes on then I know it has happened. Otherwise the first I know about it is when I open it the next day and the battery is completely flat, or if I open the lid a short while later and notice it's still running.
Rebooting always solves it - meaning that it will now suspend and resume happily again - but I have not found any less severe way to recover the situation. I also don't know what to do to force the problem.
I have found many questions about suspend not working at all - but that's not the problem here.
Is there anything I can do to diagnose please?