I agree with mike but have seen some gummy/dirty firing pin assemblies that would cause it. obviously since you cleaned it and reassembled it that is not the issue. In your first post you asked a good question about an easy way..... check the base to shoulder length on both the fired and failed to fire cases. If you have a big difference there or if the primer is sticking out the back of the case on the once that fired, that is an indication that the case is undersized or that the headspace is too long.