I am reposting because I'm getting desperate. We leave on a 3-week trip in 5 days, and LR mobile no longer works. I have LR CC, updated to 2015-12. I use MacBook Pro laptop computer, which is also up-to-date. My collections no longer show a box to the left to designate a collection to sync. My iPad Pro still has lots of previous collections but nothing new. I tested this out by trying to create a new collection on my laptop. I was not asked whether I wanted to sun with LR Mobile, and no box was created to click. When I opened LR Mobile on my iPad is said it was downloading 3 files but that never happened. I saw that some previous questions were answered with drastic measure to the "back end", but there were significant differences in the other problems So I can't do that on my own.
Did you use more than one catalog? If you do, are you certain you are using the catalog that is synchronized with Lightroom Mobile?
I have used other catalogs, but others are old (2015). Just to be sure, I opened a previous catalog and tried to sync with LR Mobile. It said I can only sync one catalog at a time and I'm currently syncing with May, 2016. I wish that were true, because that's the one I'm trying to sync. So I chose cancel.
Sent from my iPad