Henna Plants Not Thriving

Recently, I purchased six henna plants from Richters, and haven't had much luck with growing them. I read a question submitted to you at the Richters site about growing henna indoors, and I found it very helpful.
As I mentioned to you before, I haven't had much luck with my henna plants, seeing as how they dropped all of their leaves and seem to be struggling at growing new ones. I believe that I may be watering them too much, but I don't know exactly how much to water them.
My question is: How much and how often should I water my henna plant?

I think the weather must have been the culprit, because henna plants like hot, humid, sunny conditions with excellent drainage. They should not dry out completely -it is one of the things that makes them drop their leaves, but constantly wet soil kills the roots and hence the plants. Water when the soil top dries out and in cold weather put the plants on a heating pad or other warm spot to prevent fatal temperature shock.