I have the same problem on the Galaxy S8 first it's start the download and than it's stop and never run again und most of the time I don't see any photo. Sorry for my bad english
If you haven’t already try signing out and signing in.
If that doesn’t do anything try tapping on the lead photo (cover photo) in an Album and see if the restores the previews..... although that’s a pain if you have a lot of photos.
Failing that, you could delete the app and re-install (make sure all photos taken with/imported into the app are synced to the Cloud first) . I believe this worked for some users of the iOS app who had similar problems a while ago.