If you only have 1 gro-lux lamp running too little light comes to mind. Try
to fit in an extra daylight bulb (best bang for the buck IMHO). You can keep
the gro-lux, though it's not only good for plants but also for algue (ie,
keep a close eye to your tank when upgrading light). If your tank is not
bigger then 40 gallons, you couls also try to use DIY yeast CO2 (a google
search with these terms should provide plenty of info.). Don't use
fertiliser drops untill you actually SEE the plants grow, othersise you are
just acumulating (sp?) the fert. content of the water, then, when you
upgrade your lights, you will probably have an algue bloom.
Try to change 20-30% of the water each week to replenish trace elements in
the watercolumn and remove excess P and N (phosphor and nitrogen molecules).
Limmiting these will hardly effect plant growth but restrict algue growth,
in normal aquarium conditions the fresh supply of P and N would not be a
problem (fish waste).

My advice for now would be to do some waterchanges without fertilisers added
to bring levels to normal, add some root fertiliser at heavy rootfeeding
plants (use clayballs or something like that, as long as it not disolves
into the watercolumn add them near crypts, swords and such..). Then add some
extra lighting, read up about that please, lots of info on the net...
Add diy CO2, you should measure Alkalinity and PH then (again, read up
)and do yourself a favour and buy a Phosphor testkit, try ti keep P as
low as possible (feed lightly, waterchanges). Then, after you see improved
plantgrowth (and you should then) add ** moderate ** amounts of liquid
fertilisers.

Read read read, and then read some more and good luck