I can't get the slide to unlock to field disassemble my XD9 service. I've done it scores of times before with no problems. Even resorted to getting out the manual but still - the slide will not come off the lower. It feels as though it's trying to but just won't unlock and slide off. I see nothing obvious wrong ... mag's out, the chamber is clear, the trigger group seems to be working. Seems like the roll pin has dropped in the top of the slide a bit but I can't be sure. Can anybody provide some guidance??