My snapdrogons are growing well & tall but aren't blooming well?

I have 2 pots with snapdragons and some planted in the ground. They get plenty of sunlight (all of my other blooming flowers: petunias, pansies, etc.) are blooming wonderfully. However, my snapdragons in one pot have a one or two blooms every 3 weeks or so and the others have not bloomed at all, yet they are growing quite well otherwise. Any advise?