I am migrating from Aperture in Mac to LR. I have tried to create small set of collection "Sync Test" with 44 pictures in the desktop and trying to sync with the LR mobile. I have also updated a few pictures from my iPad and iPhone's LR mobile.
The results is no problem with mobile -> desktop. The desktop's collection are always stuck and saying "Syncing 49 photos" on the upper right of LR desktop interface. A empty "Sync Test" collection can be found in my iPad and iPhone but no image in it.
I tried to open the preference -> Lightroom mobile. It said "Uploads : 50 photos synced".
Any clue? This is an important feature that I need to have and value of the creative cloud to me.
System Versions -
Lightroom version: CC 2015.2.1 [ 1046594 ]
Operating system: Mac OS 10
Version: 10.11 
I have updated to the latest version and stop and restart all the collections that I want to sync, then it works.
Thank you for the update.
Let me know if you have any further queries.