Before anyone brings flames telling me to "Search, Slide not locking" I have. ALL OF THEM. Very interesting reading, learned allot and will try some new things.
But I have studied my gun and there is too much vertical slop. A generous 16th of an inch. When the mag and I have access to many is pushed up to fully seated the slide locks every time perfectly. When the mags fall to their natural position resting on the tab that engages the mag retaining notch, none will lock the slide. If it were on one or two mags I would say that the hole on the mags are over size or punched to high. But it does it with every mag. The only thing I can think is that the engagement on the right side of the mag release button is too low.
What say you self-proclaimed gun smiths and professionals alike think of gluing some material to the "Shelf" part of the release or twisting the part going around the front of the magwell between the button and the engagement to elevate it hopefully holding the mag deeper/Higher forcing contact with the slide lock when necessary. I have tried "Fiddling" with the follower to get it higher and bending the stop down closer to the follower.
Granted it's a budget truck gun but I don't want to wreck it being aggressive tweaking. I love the way it runs and plan to do some upgrades. Thanks gang.

Disclaimer and warning.....this is a comment about a GLOCK

I had to do this (make a modification) to one of the parts in my early version Glock 36, as the gun would not feed properly. (live round at the top of the magazine was hitting the frame, causing stoppages/failure to feed)

My observations were that the magazine was not being held in place high enough, because of too much gauge (open space) above the shelf on the magazine release. The amount of gauge at the location....space between where the magazine sat locked and undisturbed by me on top of the mag release.......and the magazine's position when fully inserted into the grip frame with pressure added at the bottom of the magazine (by me) .... this space worked out to approx. .070". The resulting slightly lower position of the magazine enabled the live round in the magazine to crash into the frame in front of the magazine instead of the lower part of the feed ramp. A new OEM mag release did not fix the issue, and the space was still evident.

After adding a shelf of material (Marine Tex Epoxy and insert) to the top of the magazine release, where it interfaces with the cutout in the magazine, and trimming this material to .060", all of the feeding problems went away. Doesn't sound like much, but in a sub-compact pistol design, small errors become huge.

The bottom line is, you may be able to solve your problem by making the modification you indicated. Not a perfect solution perhaps, but a solution.

BTW, since Glock management thinks that it has actually achieved "perfection", and are not still "striving" for such an illusive and unachievable goal, they had no interest in my observations.....and never provided me with any support regarding this persistent, well documented malfunction in my model 36. (compact .45) Apparently, the problem has been solved, because the 36 is still a popular model in their lineup.

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Ret. LE, FA Instr, S&W Armorer

Well, it does have a lifetime warranty....you could send it with a detailed description to the mother ship. I had some .40 Sigma mags that seemed like the slot was cut too high letting the mags sit too low, and they replaced them free of charge. Worked fine then.

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Science plus Art

I'd go with "Use the lifetime warranty at no cost to you"... Once you start monkey business to your pistol, your factory warranty service options are then very limited.