Could also be that the bottle hasn't had enough time to carbonate. You should allow three weeks or so before consuming as the beer won't carbonate properly. The sugar that is used for priming is consumed by the yeast and sits in the headspace of the bottle. It needs a little time to dissolve back down into the beer. If you open a bottle too soon you may notice that it foams up after the cap is popped off. Here is a video that better explains what you're problem may be.