I am having the same problem.
Could you please try to delete the sync.lrdata. This normally refreshes the sync process.
You can find the file here...
On Mac it's at /Users/<user>/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Win it would be at C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata
Let me know if it's working now.
Found this thread and I have the same problem. I had about 10 collections set to sync and all was well. Went to add another yesterday and it stalled trying to do the sync. Tried the suggestion here and deleted the sync.lrdata folder. The sync started from the beginning and I guess reconcilled the existing pictures but stalled at the same number again. So that didn't work.
I then saw on another thread someone suggested the delete all from the preference menu. Did that and was able to add the collection that would not sync previously but now when I tried to add the other 10 collections back that were originally there it stalls. Turned off all the collection syncs except for the one that made it ok and one collection of only 7 photos. Sync is stalled on 7.
What's going on, anyone find a way to fix this?
Could you please send me a LR Desktop diagnostig log - best as a private message with a downloadable dropbox link.
You can trigger the log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button
Sent you an email with links.
This is a problem for me to. I have managed to get it to work by clicking on the download icon on the left although you may hav eto do it a couple of times. I am removing the collection and then only doing one at a time to get them on my ipad.