It could be any number of things. Not all LED fixtures like to be dimmed. If they are DMX units, the flicker might be due to an unterminated DMX line (or other issues with DMX line). Also, there are a lot of cheap LED fixtures out there. Many of the "cost effective" ones do not perform well, and come no where near achieving the rated life hours they are said to.If you are able to offer more information as to what the fixtures are, how they are controlled, and when you seem to experience the flickering, perhaps we could offer more specific advice.