More then likely it is the built in "stops" on the follower or base plate.
There is a very fine line on designing a mag that will not allow ammo to "bounce" out when dropped.... and allowing enough room for the 30rd loaded mags to seat and or fit the full amount of ammo in.
I have fixed any mag seating issues by shortening the stops. Last time I described how.... I got blasted for telling the OP how to ruin his mags... all of my altered mags function perfectly. And I do it to all my mags, I remove about a 1/16 of an inch off the stops.
If you want to try it , IM me. ..... If you want to visually see what I am talking about ... fully load a USGI metal mag and a Magpul.... then depress the ammo, you will note the ammo in the USGI moves down quite a bit more then the Magpul.
I have had NO issues with shortened stops in all styles of Lancers, Magpul and assorted others. As a matter of fact, Lancer used to show how in a video on their webpage.