The first thing to check, is to ensure that you are providing enough light for your plants. If you don't have enough light, then all the ferilizers you add won't make a bit of difference. What size aquarium? How many watts of light over the aquarium?
You'll also want to ensure that you are actually dealing with true aquatic plants. Many of the chain stores sell plants that aren't aquatic and will eventually rot and die in your aquarium when kept submerged long term.