I have put about 300 rounds through my new XD-40 and I have been having issues with the slide not locking back after the last shot. I have used 165 gr. Blaser FMJ and the Remington Mega Pack FMJ ( sorry, i dont now the gr. size). The problem seems to be happening about 2 out of every 5 times. Any suggestions would be great, I am new to handguns. Thanks