Bought a NIB 107UR a few years ago, immediately form 1'd it.
Shot it one time to test after paperwork approved and work completed, 60 rounds, slow fire, couple double taps, functioned great. Used a lightly used 30 round Bulgy circle 10 waffle mag and surplus brass case, corrosive chezch ammo, cleaned and put away.
Took it out for the second time yesterday, used new 30 round Bulgy circle 10 waffle mag and also a new polymer slab side 30 round Bulgy mag and some Russian, non corrosive unmarked small white boxes of ammo. Cases were steel and green and grey.
Loaded up each mag 2x. Experienced the same issue aprox half way through each mag.
Issue, bullet was not chambering and seemed to hang up on feed lips.
Pulling the charging handle back a little and releasing sent the round back into chamber and everything was good to go. I was hammering it pretty hard with some rapid fire as well, although that shouldn't have been an issue.
Not sure why this is happening?
Do polymer feed lipped mags need to be broken in?
Under gassed (probably not as this only happened 4 times out of 120 rounds)?
Under powered ammo?
Bolt carrier hanging up on rail, hammer, etc...???
Any thoughts or ideas?

go shoot about 500 rounds through it. It's a new gun and could be as simple as the bolt carrier and hammer need to be worn in. The only other thing I would check would be mags a vertical movement while seated.