If you primed with something other than dextrose (corn sugar), then it's possible that there weren't enough fermentable sugars in there. I had this happen to me recently---I wanted to use DME to prime, and so I used laaglander brand DME...which (now I know) has a low fermentability ratio. Turned two batches into nearly-flat brew. Had to uncap, drop in carb tabs, recap. Suxxorz. So that could be it.
It's also possible that you didn't stir the priming sugar into the beer well enough during bottling...and that, while some of the bottles are undercarb'd, other might end up gushing.
It also might just be taking a long time. What happens when you pop the top? A loud hiss? Anything at all? How much carbonation is there? How long have they been in bottle?