I would start with the ammo. Is this reloaded ammo or factory? If reloads, then get some factory ammo. If factory, what is the weight of the bullet? Assuming you have decent factory NATO ammo, I would use the factory mags. I always take apart and clean the mags. I might suggest this, except you are reporting issues with the pmags. Hence, I would, as suggested above, strip the gun and check it out. Give it a good cleaning and generous lubing. These guns prefer to run wet. If it appears dry, I would assume this is the problem. Hence, strip, clean, lube and try again with factory ammo. If still have a problem, then you may want to contact DPMS.
Cheers, Steve