Brush roll slows/stops turning

Question

The brush roll/beater bar on my G4 won't turn if I try to lower the vacuum to the necessary height for it to clean the carpet well. I thought it might be time for a new belt, so I replaced that. No help. Then I scuffed up the wood under the belt because it seemed almost polished and shiny. That still didn't fix it. Then I thought maybe it was only because my old, crummy quality carpet used to get sucked up against the rug plate if I slowed too much. But now I have new carpet and it still won't spin if I lower the vacuum more than two clicks. Do I need a new brush roll? or is it something in the motor? Thanks!