I just purchased a brand new M&P Shield 9 and I am unable to lock the slide manually. I took it to the range to try to loosen it, fired 50 rounds and had 5 FTFs. The slide locked fine when the magazine emptied, but I still cannot do it myself. I really want to clean it, but I can't strip it without locking the slide back. Before I purchased this gun, I went to the range and rented one to make sure I could handle it properly and I had no issues then. Racking the slide is no problem; it is just the slide stop will not push up and feels like there is something blocking it. Has anyone else had this issue or have any ideas about how to fix it? Any help would be appreciated