LR no longer syncs with LR Mobile

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.