King Arms Thompson 1928 has feeding issues w/heavier pellets

OK, so for a couple of months now, I've owned a King Arms Thompson Model 1928 (the one with the charging handle on top, not the M1-style charging handle that most 1928 airsoft replicas have), and barring an instance where it had to be sent back for repairs after running away on me during a game, it's been treating me well. However, today, I used .28 gram pellets with it instead of the .25 gram pellets that I usually use in my AEGs because I felt that since it was already doing an average of 330-335 FPS with the .25s, then going up a little in weight wouldn't hurt. However, because of that, it's been having feeding issues. Before anyone asks: 1. I have been winding the drum mag all the way, and 2. I haven't tried the three hi-cap stick mags today that I bought as backups.