Take a look at the little ledge on the left front of the magazine follower. It pushes up on the slide lock when the mag is empty. Mine wore down or chipped. Then the slight slop in the follower after the recoil of last shot releases the lift on the slide lock. It works okay when you insert the...
Got a reply back from S&W in about 3 hours. Said to send the mag in and they would replace it.
And, no, spring is not in backwards. You can depress the follower with a feather.
I've had similar problems on a new BGT380. Figured out it was both a weak magazine spring (this also caused miss-feeds of the last round) and the corner of the magazine follower that lifts the slide lock inside the magazine well is worn or chipped, so sometimes it works and usually doesn't work...