I don't know, I know people that grow moss in tanks with high light without any problems, and its not like you would want to go out and get a new light fixture with LESS light just to grow your moss. It may be you piled the moss on too thick. Also if it was commercial grown moss, it was grown emersed and it takes a while for it to grow out under water. I have found moss can be very slow to grow out in the begining, like a few months before something starts to happen. Once it gets going it grows much faster. I would just give it more time.