It sounds like the yeast are just dormant or tired. Either that or you're not adding enough priming sugar. Personally, I would add just a simple pack of dry ale yeast a few days before bottling. That way you know you have enough healthy yeast to carbonate your beer.
As for your current batch, you can open the bottles and add a couple of grains of dry yeast to each. It's kind of tedious, but it might help them carb up.