I bought a couple of used XD9's recently, a 3" and a 4". The 3" operates as advertised, but the 4" has a problem. The slide does not lock open after the last shot. I thought something was wrong when the slide release seemed to be spring-loaded in the wrong direction. The release has to be manually pushed up into the recess on the slide to lock it open at any time. Then, sure enough, it didn't lock open upon firing the last shot. Is this a common problem? Broken spring? Assembled incorrectly? Is it something I can fix myself? Thanks for your help!