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)
• 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.

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)
• 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.