I've had a problem getting my collections to sync for over three months now. Lr Classic used to magically sync my dozens of collections, and over 5,000 files, and did it without any hiccups for several years. Then, suddenly in late July it just stopped working. It would just hang trying to update any new items in any collection, and still does.
I have called support, opened two separate tickets, and followed everything they asked me to do, plus everything ever mentioned here at least three times, and still there's no fix. I even exported a large collection of my portfolio as DNGs, created a new Adobe account, created a new macOS account so everything would be in a clean state, and tried syncing about 1,000 of my photos from that account. LrC still got stuck with 200 photos remaining to sync. Why can't the entire sync database just be reset so we can just start from scratch?
I'm frankly at wit's end. I totally fell in love with LR Classic sync, and used it across my macOS desktop, my iPad, and even my iPhone, and it was working brilliantly. It was so magical, I built my whole workflow around it for my portfolio, my students, and even my clients. Heck, I even built my whole website with it in Adobe Portfolio! Now it's just dead in the water, and I'm stuck, and feel like Adobe doesn't even care.
If anyone has any ideas how to get this to work again, I'd sure love to hear them.