My Dept just went to the 21SF in August, we found some of the guns wouldn't drop mags all the time.
We found that some of the mag wells were tight....if you press in on the sides of the grip you can see it flex closing the mag well a little.
Our fix is to cut up some file folders in strips slightly narrower than the front to back width of the mags.
Take two or three of the strips and wrap them on the side of the mag over the feed lips and over the other side of the mag.
Then insert the mag as far into the mag well as you can.....it will be tight and won't go all the way in.
Leave it that way overnight then remove the mag and try it to see if they drop free if they don't just do it again using another thickness of the file folder.
This opens up the magwell allowing the mags to drop free.
It works well and if over time they start to get tight again just use the folder strips again.
I had to do this to my issued gun which I've had about 6 months and the mags still drop free like the day I did this.