Maybe Tom, did you round off the top back edge of the follower slightly, so the cartridge rim won't catch on the top of the new follower?
I'll bet that's the problem! Just add a slight rounding (or a 45 degree chamfer) to the top back edge to allow the rim to slide forward when the bolt hits it. I found that the rim would catch on the follower and hold if this edge was sharp. That's why the other rounds feed normally but the last one won't because they don't come in contact with that edge, only the last round does.