Thats a good question. Maybe thats what I am doing wrong. Too much light. I have about 4 species growing under T5's. My C. cordata grew well for a little while and then started deing off. I moved it to a darker spot and is slowly recovering. My C. willisii has been doing well under bright lights, while my C. albida did really well until a few months ago ( i suspect the down turn is more due to predation by mites than anything else). My C. parva grows slowly but seems to like the intense lighting. My Lagenandra ovata, grows very slowly but seems to dislike the lights as the leaves have turned pale green. I'll probably move that one. I keep an L. thwaitesii in darker conditions and grows better than my L. ovata. I use 6 T5's 3 are 4100K 54 W, and three are higher spectrum (blueish lights) also 54W. Hope that helps.
It depends on a number of factors like species, light intensity, duration, and temperature. My windowsill growers outperform my basement grow chamber plants until it gets too hot.
I have several crispatula balansae and wendtii emersed in a 10-gallon tank in a south window. They get a little shading from a couple of Echinodorus species in there as well. They grow perfectly fine with no problems. I do have to vent it during the summer to keep the tank from overheating though.
On the other hand, I have a small terrarium on my east facing kitchen window sill with a C. x willissii that thrives fall through spring and really suffers during the summer. Although the light is not as intense or as long, it is not vented and I do see leaf scorch during the summer.
These and many others are also in my basement under regular shop lights about 18 inches above the plants. They show very slow but steady growth spring through fall and stop during the winter when the basement drops into the low 60's.
I used to put 4x T5 lights for my emerse crypts and later realised that it create too high a temperature and some plant leaf edges are "burned". Another problem arised when the water droplets from the roof fell on the leaf, they seem to create a magnifying glass effect on the leaf and burn a hole through. Now I cut the light down to 2 and they seems to do better.
I find that a balance need to be strike. The higher the temperature, the more evaporation and the higher the humidity. However, the increase in the temperature and humidity is not proportional. When the humidity super-saturated, the further increase in temperature is detrimental to the plant.