Shared album photos issues

My son shared a photos album with me, which I accepted on my iPhone and can see all the photos in the album. On my iMac I can see the shared album in the list, but there's no photos in it. I have the invitation to accept in Activities in the Photos app. I'd of thought it should show accepted anyway, but clicking on Accept has no action. I wondered if it needs to have an iCloud sync, but it's been over 2 hours now.

Quitting and reopening Photos doesn't fix it.

Ideas anyone?

TIA

Peter