At what temp are the bottles sitting? If it's under 70F then you need to place them closer to the heat...i.e. water heater, for a time.
How much priming sugar did you use? Better yet, what type of brew are we talking about?
I've had brews sitting for 6-8 months that never carbonated until I placed them in the fridge...go figure...
Get back to us so we can try to narrow down your problem.