I recently upgraded to a new iMac. During two Zoom meetings with 20-plus members since then, I was unable to use the arrow keys to advance my shared PowerPoint slides. Instead I had to use the ultra-sensitive touch pad atop the Magic Mouse.
This problem did not occur the night before last when our guest speaker and I practiced his presentation. Neither does it occur when I practice using my laptop and iPhone as other meeting participants.
Curiously, the arrow keys worked for the first couple of slides last night then quit working for the rest of the meeting.
I did not have this problem with my old iMac running Catalina.