I diagnosed the cause of my issues. Looks like the port at the top of the mag is not sealing with the port in the slide. If I put upward pressure on the mag to seat it higher, the pistol works great. If I pull down on the mag I just hear a poof sound and the slide does not operate. Need to figure out how to either seal that better or keep the mags higher. FWIW, this happens with both a We mag and a Tokyo Marui mag.
Any ideas?