First, as someone before me said, disassemble them and:
- clean inside of mag body, then lube the inside with a dry lube
- also look for burrs or plastic flash on the follower - remove if found (either an xacto knife, or file / sandpaper
- re-assemble and finger cycle the follower down, this does 2 things:
a. checks that follower keeps initial orientation being depressed
b. checks to see if the follower hangs up - specifically on the upper cut out. I have experienced, as have others here, the follower lower edge get caught (or drag) of the cutout sharp edge - I check for this with every mag find most do even a little. I will then take a small file and slightly bevel this area of the (removed) followers lower edge so it slides right past the cutout.
- load full and store that way for awhile - periodically unload and reload to cycle the mag. Shoot it a few times too and see then how it performs.
While I can, with thumb effort, hand load other guns mags - they all get tougher near capacity. Doesn't take much of that to get thumb tender. Uplula in your range bag is faster with less damage to your fingers - best $40 you'll spend (on gun "stuff")