why do my icloud photos not show up on my mac air

I had a bunch of photos on my iPad. Usually they show up on my Mac air, when opening up the photo app. Didn't show up, so opened iCloud photos and they were all there, but I still can't get them to show up in photo app on my Mac air. I followed instructions on how to rectify this, and when a transfer began it showed over 4500 photos were transferring. I am using MacOS High Sierra 10.13.6. It has always worked correctly in the past. Ideas?