There have been a number of recent threads on this exact topic. Do a quick search and you will find lots of useful information.
Usually, undercarbonation occurs because people don't leave their beers long enough in the bottle (3 weeks is the minimum) or they have their bottles at too cool a temp (bottles should be around 75 degrees for the first week or two). The other common problem is not sufficiently mixing your priming sugar into the beer.
Given the time you have had your beer in the bottles, it sounds like the second or third issue might be most relevant to you.
If you are worried that you don't have enough yeast or priming sugar in the bottles to carbonate them, it is possible to add more of one or the other or both. See here for some details:
http://www.homebrewtalk.com/f13/going-guerrilla-these-fools-31582-post310490/#post310490