Why can Universal Clipboard only work on the Mac I last signed into iCloud on?

Hi all,

I need some help...

I have an iMac Late 2013 and a MacBook Pro Late 2013. The iMac is running on the latest Catalina and the MacBook Pro on the latest Big Sur.

Handoff works well for both, however, the universal clipboard is a different story. Initially it was working on my iMac to iPhone and not the MacBook to iPhone.

I signed out of iCloud on the MacBook and logged back in. This fixed the problem and the copy and paste function was working fine on the MacBook but in doing so caused it to stop working on the iMac.

I then signed out of iCloud on the iMac and logged back in and this fixed the issue on the iMac but you guessed it caused the copy and paste function to stop working on the MacBook to iPhone.

Oddly I have always been able to copy and paste from iPhone to either Macs.

Is there an issue with universal clipboard only working on the last Mac that was signed into or is it to do with the fact that I have 2 different OS versions signed in at the same time. I cannot update to Big Sur on the iMac unfortunately due to its age.

I have also tried all of the other troubleshooting tips including deleting bluetooth lists etc

Any help would be greatly appreciated.