Temperature can often be the reason. I had a Christmas beer that didn't carbonate because my basement was too cold. I simply took the bottles to a warmer place and they carbonated in the normal time.

Here are some reasons a beer won't carbonate:

 - **Temperature**: If the beer is too cold it can put the yeast into hibernation. Warming up the bottle might be all you need.
 - **Tired/stressed yeast**: if your yeast was old or you didn't have enough yeast, or the beer is a stronger beer, there might not be enough healthy yeast left to carbonate the beer. You can add more yeast at bottling time ([Related Question][1])
 - **Improper bottle seal**: check to be sure the bottles are correctly sealed. If there's any gap at all, the C02 will escape as it's being created
 - **Forgetting the carbonating sugar**: You've already thought of this, but I added it for completelness.


  [1]: http://homebrew.stackexchange.com/questions/148/when-bottle-conditioning-when-should-i-re-yeast-and-how-much-yeast-should-i-add