Definitely what Doc just said.
Case in point. I built an MK12 in 6.5 Grendel that I just took to the annual desert meet-up back in November. I took 3 different makes of 10 round magazines just to function test them. On top of that. I bought some 30 round mags locally once in AZ. 2 out of the 3, 10 round mags ran just fine. One didn't like to run. The locally sourced 30 round mags were nothing but jam-o-matics.
So always try different mags in any rifle, just to rule them out. After all, they are one of the moving parts of the rifle.