Magic Keyboard won't connect to iMac via Bluetooth but will connect to MacBook Air.

My Magic Keyboard suddenly will not connect to my iMac using Bluetooth. It does connect via Bluetooth to my MacBook Air. It works via USB with my iMac.

My Magic Mouse does connect to my iMac via Bluetooth so Bluetooth appears to be working on both my keyboard and iMac.

I recently updated to the latest Ventura OS with no known issues but this problem did not start then. I've turned bluetooth on and off, restarted my computer and reset PRAM which has not helped.

Any ideas what the problem is and how I might resolve it (other than connecting my keyboard via USB)?

Thanks