I have the PWS MK109 300blackout on a PWS pistol receiver and I took it to the range this Friday. I had the same problem with the exactly the same ammo, Remington 300 BLKOUT 7.62x35mm 115 gr Close Tip Flat Base. I tried 3 different mags, Pmag, Victory and GI, and it had the same problem. I took out the carrier bolt assembly, and re-oil it and still the same problem. Since I have two PWS pistols, I swapped out the receiver, and had the same issue.
Another post indicated maybe problems with the spring, and maybe use a lighter spring. Here is the link. viewtopic.php?f=136&t=79229
I only had one PMAG that day, and based on another forum, I need to try out multiple PMAG to figure out which one the 300 BLKOUT ammo likes. I'll give that a try this week, and also call PWS to get an answer. If you get a reply from PWS, can you also post their reply.