I had a similar issue with my vacuum a while back. Even though the belt seemed okay and the suction was working, the beater bar refused to spin.
It turned out the gear the belt sits on was entangled with hair and threads. I unplugged the vacuum, opened the beater bar cover, and removed the bar and belt.
After clearing the debris from the gear, I reassembled everything, and voila! The beater bar started spinning again.
As for the standing issue, it might be separate, but always worth checking if there's any wear on the vacuum's base or locking mechanism.
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