So the slide lock functions correctly under hand power, but fails while firing.

1.The slide is not traveling fully to the rear due to under powered rounds (doubtful) or a bind (you should be able to feel it when you work the slide by hand).
2.The lock is disengaging during the recoil cycle. Inertia, thumbing, or the leading edge of the lock is angled poorly are all possibilities.

Take a pic from the side with the slide locked back.