Could you try to remove the sync.lrdata file and restart LrD. This would restart/refresh the sync (metadata only).
Removing the Sync.lrdata might fix whatever the issue is. Or, at least, might trigger it to occur again. So remove the Sync.lrdata, start Lr, and then wait until it either syncs or gets stuck again and create at this point a new diagnostic log which can hopefully provide some clues.
You can find the sync.lrdata 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
You can trigger the diagnostig log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button
Please send me a LR Desktop diagnostig log - best as a private message with a downloadable dropbox link.
Btw. Are there any sync errors listed in Lr Desktop?
Cheers, that did the trick...all 4 collections are now re-syncing
Same thing happening to me.
I tried to locate the file on my Mac but finder says no folder can be found. This is the file path I used.
I have 5.6 also. I did notice when searching for all files related to LR that there was a file called 'Mobile Downloads.lrdata' but no other with the extension of .lrdata?
Should I remove that one and follow your instructions?
Did you replace <user> with the name of your user/login account on your Mac?
Thanks, that helped. Just deleted it and I'm loading LR again to see if it syncs. Thanks again :-)
I have LR Desktop 5.7 and LR Mobile. I had the same problem today. I followed your steps of deleting the Sync.lrdata file. In addition, I deleted the LR Mobile app on my iPad and reinstalled it.
These two steps solved the problem. Thank you!
Thank you for your insight!
I use Lightroom CC, and with the additional step of quitting and relaunching Lightroom CC, your solution worked like a charm.
Hope this helps other with the same problem.