I'm still new to this game, but I too have noticed the center burn you describe. I get the even burn I want by tamping gently and often, but with the pipe nail angled such that the bit that is closest to the edge of the bowl is lowest, thereby forcing the tobacco to cone inward. This seems to keep it pressed into the ember whilst still allowing me a good cool smoke. I've gotten where I mostly only need to relight if I don't pay attention.