There are a few things you might try to keep him in place. Put something like a strawberry container over him until he attaches, or pile up some rubble pieces around him. He's probably on a walk-a-bout. They seem to do this ever so often. I don't think they are necessarily unhappy, I think it's just the forces of nature at work.
Addressing the lighting question, old bulbs may have shifted their color spectrum and the nem dosen't like it. If it's an old tank double check your nitrates. High trates can cause them to want to change their place to find conditions like they were previously in.
Or, could just be a walk-a-bout. I've had them roll and tumble all over the tank. And yes, there will probably be some casualities if he continues with his unruley behaviour.