Have you connected the front panel's header right?![]()
Are you able to provide some pictures? Hard to suss out what is wrong without looking at the problem. It could be many things. Your placing the IO shield upside down, Something it caught on case stopping it going in or the IO shield hole has burrs on it from the factory stopping it going in.
Pictures would help a lot in this case otherwise there isn't much we can do but guess and you get frustrated.