Here is some more general advice I posted to another thread regarding bottle conditioning, since the topic seems to come up quite a bit:
1. Assuming you primed correctly, you want to carbonate for at least a week or two at about 70 - 75 F, then another week or two (could be at cellar temperature) to ensure proper carbonation. Sometimes carbonation will take 1 week, sometimes it will take 4. The yeast strain, recipe, length of time since the brew, fermentation temperature, etc. are all variables that contribute to the carbonation time.
2. If you haven't carbonated the first week or two at warmer temperatures and the beer is now flat, then gently swirly your bottles to re-suspend the yeast, and move the bottles to a warm spot for at least a week or two.
3. If that doesn't help, then be patient and leave them for another two weeks.
4. If that doesn't do the trick, you probably have a capping problem or yeast problem. Put a balloon or condom over the neck and cap of the bottle, give it a good shake, and see if any CO2 is leaking out. If so, proceed to step 6 (re-priming).
5. If faulty caps weren't your problem, then you might have to try adding fresh yeast to the bottles. A few grains of dry yeast are all that is necessary. Just make sure to chill the bottled beer first, or adding the yeast will cause gushers.
6. If that still doesn't do it, then you have under-primed. You will need to shake the bottles to release any built-up carbonation, carefully vent the CO2, chill them REALLY cold, and one-by-one remove the cap and add some carbonation tablets to re-prime. Recap and start again. Don't skip the venting and chilling step -- otherwise you will find out why very quickly!
** Make sure you don't jump steps because if you do, you might over-carbonate your beer and create bottle bombs. Not only will it make for bad beer, they are seriously dangerous.