I'm leaning toward an imbalance in nutrients and/or lighting. I'm probably not going to be able to give you any answers but if you give us info on the tank perimeters someone may be able to help.
Lighting - type, watts, duration of light, sunlight or not
have you tested the water? LFS can do it if you don't have test kits. Make sure you tell them the tank has plants.
Is it light, moderate or heavily planted
Fishload
filtration
CO2
are you using: ferts (details) carbon source (like florish excel) carbon or ammochips