Photos not syncing

I recently upgraded from a MacMini 2011 to 2018. I used to have my photos on an external HDD but I now have sufficient space I have everything on one SDD. I copied everything across from my old machine to the new but my problem is that I just can’t get the new MacMini to sync with iCloud on photos. It does for everything else, so it must be something related to how I set it up on my previous machine. I have done the usual things of logging out and back in again but to no avail. I’ve had a look across the Apple help but none seem to really assist - any additional ideas please? thanks.

If you attach the old external drive, will it sync then?

iCloud photos is known for its very slow sync, due to facial recognition, metadata and so forth. You might just leave things running for a while and see what happens.