My FX high capacity mag once in a while jams. Will randomly not feed a pellet, and I get a dry fire. Today i checked the cover, as I suspected the pellets might catch into something. Took a pellet and moved it allong the inside of the cover, and when it came to the feeding hole, it catched the edge.
There was a tiny burr there not detectable by eye.
Took a dremel tip, and removed it by hand, and afterward I could move the pellet without it catching. Might be worth checking if some of you have the same issue