Basically, there are so many reasons why sleep could fail that it i impossible for us to give you the reason why in your case. You could search Google or Bing for things you could try to isolate the cause, but there is no guarantee you would succeed. So the most practical solution is to work-around it. Which means:
a) Disable sleep and hibernation modes (as you have done).
b) Save cost by switching the machine off when not in use.