Glock mags are notorious for sticking and not dropping free - do a Google search if you think I'm crazy. There are several reasons for this, and maybe we'll make a video explaining, but here's what we've tried to do with the S15 magazine to mitigate the issue.
First, it's worth mentioning that there are AT LEAST two generations of Glock 43X/48 frames. When the black slide guns were released, Glock also changed the frame. We should do a video on this as well, since this frame change set us back several weeks on the S15, and unless you REALLY look for the changes, you won't see them. Back to drop free - we spec'd the width of the S15 mags so that they should drop free in gen 1 and gen 2 G43X/48 frames based on the data we've collected. Our manufacturer, Check-Mate, runs mags through a drop free gauge with our spec during assembly as part of the QC process. Between the at least two generations of the frames and multiple molds for each gen, there's a fairly wide tolerance range at play here.
The second part of the equation is surface finish. Friction is the enemy, so a slick surface finish is important. The magazines come lightly covered in oil from manufacturing, so it's possible that there is dirt or other residue sticking to the outside of the magazine. We recommend cleaning the outside of the magazine with your favorite gun lubricant (we prefer EEZOX) to remove any dirt or residue from manufacturing and packaging.
We have six or so test guns, gen 1 and gen 2, and our S15 mags drop free in all of them. However, if despite our efforts your mags don't drop free, there is a simple fix - a hair dryer. Shooters for years have been using painters tape on OEM mags and a hair dryer to slightly stretch out their Glock frames. Do a YouTube search for details.
Hope this helps!
-Brandon