I've had the same issue with my VFC PDW using PMAGs M version and EMAGs and pretty much other brands like my Invader Magazines too.
The stock KAC PDW mags were fine did not fall out.
My issue was when I start shooting the mag would just drop after a few rounds.
I ended up shimming the magwell with Real Steel Grip Tape for pistols the rubber kind not the sand paper kind. Shimmed the rear and one of the sides inside the magwell. On top of that I replaced the spring as well with a stiffer spring. Believe it was a King Arms Spring.
Did both of these mods at once and never had a single issue again with dropping mags.
The only problem with shimming the magwell this way is that a lot of other standard M4 mag would not fit.
But I'm fine seeing how all I really use are PMAGs and EMAGs
edit: here are some pics
The grip tape is not going to come off... its on there pretty solid unless you peel it off.