Some plants require a lot of light and a lot of fertilisers and CO2, but others do fine with no fert/CO2 and a minimum of light. Telling us what plants you have may help us rule out possible causes.
I had heavily planted a 100 gallon a few years ago and after they started to grow VERY VERY big they started dying... maybe they drained my tank of all its nutrients, either way you should try some CO2.