Oh! Fun observation! If I take the entire brush assembly out, it appears to work just fine, (only there is no cleaning brush).
Just reassembled the brush and toyed with the left-right slop and now it works. I’m betting there was something binding on the brush and the brush guard that prevented it from rotating. Well, hope this is a lesson that others can use. And it’s a lesson to myself that once I post a question online, I seem to find the answer myself soon thereafter.