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.
I was in the same situation with the synching problem but happened on a fix last week. I'm a CC member and have LR installed on two machines. My desktop is my production system with collections and sync set up. I had downloaded a second copy of LR for my MacBook Pro to use for quick photo edits, but didn't set up a second catalog, etc. However, I did sign into my CC account on this second install of LR. Once I signed in on the second LR installation, the mobile syncing promptly stopped working. Once I signed out of the second install of LR and signed out and back into my LR Mobile apps on my iPhone and iPad, it started working. A couple of weeks later and its still good.