Unable to screen share from 1 10.15.7 iMac - does work or another ...

I have an iMac which is running 10.15.7 and I'm unable to connect to it via ARD or the Screen sharing app.

I have another of the same machines and same OS and it is working perfectly fine.

Ive tried the usual turning on and off screen sharing/Remote management - and that has no effect. Ive checked IP's, connections, changed the computers name, changed network ports, deleted the machine out of the ARD machine list and now cant even see it when searching - it was there from previous scans some time ago - basically the machine is seen in "the finder" - Connect to Server / Browse - but it isnt connecting just coming up with the "Unable to communicate with “aaa_AMAC_Nick" error.

It seems like something is just not allowing external authentication seeing Im not given a chance to put in an admin login ...

I'm aware some security stuff was changed/tightened - but why are other macs with exactly the same OS and settings working OK ?

Cheers

Mitch