My brand new P229 Elite had this same problem. The first 8 times the mag went empty, the slide failed to lock back. The ninth time it locked back and the tenth it did not. As it was my first pistol, I assumed it was ready to go right out of the box and did not clean or lubricate it prior to taking it to the range. It has since been cleaned and oiled, but I have not been back to the range with it yet. I anticipate/hope that it'll be just fine. If not, I have a lifetime warranty!