I discovered the other day that grain dust is NOT the culprit. My second roller developed so much drag that I took it all apart and cleaned it again, and re-assembled it. It was still stiff. I began unbolting it from the base, and noticed that the roller had freed up to the extent that it would free spin and coast! I fiddled a bit, and ultimately left one bolt slightly loose, and it worked like a dream.
H.W.
Won't this mess with your gap. or am I misunderstanding what you're doing? I'd think a loose screw would allow the roller to get out of alignment.