Your yeast may not foam for 2 reasons: 1) You are using instant yeast. There will be less of a reaction from this type of yeast, but it should bubble a bit. 2) Your yeast is dead. This may occur because the yeast is expired, you haven't let it sit long enough, or your water was much too hot and killed the yeast. In this situation, I recommend you re-start with some fresh yeast, sugar, and warm (not hot) water to let it bloom again.