Wife bought a XDm 9mm (4.5") barrel last weekend. Has been shooting a Glock 23. 200 rounds later, the pistol has failed to lock the slide back after the last round. She said this is happening about every 3rd mag. I have not shot the thing yet as I am recovering from a cracked tailbone and been laid up. Could this be a mag problem? All ammo is PMC 115 grain? Have not seen this in my Glocks or 1911's. Appreciate the help as I drift on a nice perc cloud.