Yesterday, I noticed my auto plunger was getting weaker and weaker as I played. I increased the plunge power from 175 up to 200 in the settings. A few balls later, I had to increase it again. Finally, with the setting on auto plunge maxed out, the plunger was firing super weak and wouldn't even get the balls up the ramp.
I lifted the playfield to look under the apron for any obstructions and found none. I reseated the playfield, set the plunger back to factory default "200" and entered the coil test mode to repeatedly plunge balls to diagnose the issue. The auto plunge successfully plunged all 20 test balls perfectly up the ramp and into the van -- four go into the van then it ejects them when testing.
So, I put the glass back on and haven't had an issue since. Maybe it was me monkeying with the auto plunge mech while checking for obstructions fixed whatever was wrong with it? That is an odd mechanism that I hope doesn't come back to give me more trouble.