Have you tried a different magazine? I like to start with inexpensive and simple. If you are SURE you're not riding the slide release with your hand, the next in line for me would be to swap out magazines. The Slide Lock back is caused by the follower in the magazine pushing upwards on the slide release lever causing the it to move upward... where the slide lock catches the groove in the slide... locking it back after the last round. If that's not functioning correctly, you won't have your lock back working as designed. A mag issue, not a firearm issue. I look at magazines like I do shoes and tires....when they are done, they're done. I work in from there. Your slide stop may be wore out of spec. Replacements can had for under 20 bucks from Numrich.
I'd experiment with cheap and simple before making the gunsmith's next boat payment for him . If you don't have other mags, can you borrow a couple?