I may have figured this out. I mean, there are only a few things that need to work right in order for the slide to lock back, so it just had to be something a guy could fix himself, right?
Out of my 5 mags, all with the identical follower (same patent # stamped on each one), only one would lock the slide. I really had to study it to discover what was different about it from the other 4. I finally found it. At the "V" bend where the upper and lower parts of the follower come together, the working follower had a tighter bend in it (sharper angle) than the other 4 did. (see photo in post below)
I have some pliers with parallel jaws that come together perfectly flat, so I took each follower and a nice crisp angle at the rear of each one. I had to squeeze these buggers pretty hard, as the steel in these followers springs back to its former position pretty well. This process left each follower a little "flatter" when I was finished.
After replacing each follower on its spring and back into the magazine, I then opened up the angle just slightly with a small wooden dowel (about pencil size). I'll be darned if every one of these things isn't locking back the slide now.