By far the most common deficiency is CO2. If you are using pressurized CO2 you can try increasing the bubble rate quite a bit, since you have no fish to worry about. If you have DIY CO2, try using another bottle, to get more CO2.
The 65 watt PC lights most commonly sold in LFS seem to all have very poor reflectors, so you may have only low light, possibly very low light. If so, it might be the light which is limiting the plants growth.