Got a new 9mm shield. Took it to the range last weekend. If using a full mag, 7 or 8 round, the slide would not lock back after the last shot. Maybe did it 1 or 2 times out of 20 mags. Load just one round and it would lock back less than 50% of the time. Tried this 6 or 8 different times over 30-45 minutes. In each scenario if I pull the slide back manually after last round the slide locks back properly. Put an empty mag in, pull slide back it locks back properly. Gun was cleaned and lubed properly before range trip. Was using Winchester white box 115 gr brass ammo. Same ammo was used in USPSA match 30 minutes prior in M&P FS 9mm with no issues so I don't think it is ammo related. Anyone else experience issues like this. Do I need to send it back to S&W.