I'm making some good progress with the factory mags.
After loading the mag all the way, I thumbed the rounds out until I got to the first one that wouldn't pop up on its own. Then I used the LULA loader to repeatedly press the remaining bullets in the magazine down one position, and then let them come back up. Then I pressed them back down again, and let them pop back up again, repeating the process 100 times.
Basically, there are "tight spots" where the follower is binding against the inside of the magazine. By repeatedly moving the follower up and down in the rough spot, I'm either smoothing out the finish on the inside of the mag, or I'm polishing down the follower.
Regardless, I can now hand-load all 13 rounds into the mag, and all 13 rounds will pop up when I "thumb" them out. They don't all pop up with the same force, so I still have a little more to do, but I think this is going to work.
I'm still going to look at getting a Mec-Gar mag once the buying frenzy calms down, though.