So here's the fix:
The manual suggests lubricating the horizontal ball rod and the upper pulley wheel axles every 60 days. But for some reason, I can't find anywhere that it mentions the two vertical belt/pulley rods.
These were all coated w/gunk, maybe old lubrication, and the black bracket was getting hung up on one of them. I cleaned them both with isopropyl alcohol, re-lubricated with silicon lube spray, and they are working great now.
As Frank Zappa used to say, keep it greasy so it'll go down easy.