Lightroom Classic Sync up to its old tricks

I hadn't been on Lightroom classic for over a week. 
I uploaded several photos from my phone to the cloud and they instantly synced to the cloud and also with Classic. 
I then tried to sync 17 photos from Classic to the cloud. I started last night and the blue sync button just keeps going round and round and round. After an hour last night I switched it of and resumed this morning and it's being going for another hour and still 17 photos to sync. 
Just as a test I uploaded another photo from my phone to the cloud and that instantly synced with classic.
So the problem is Classic to the cloud. 
Now I say "up to its old tricks" because I have noticed this same behaviour in the past. Whenever I have gone for a significant period of not using Lightroom the first syncs take forever. It's as if I have been put at the back of the syncing queue.