I have been having the same issue and its killing my workflow.
This is October 2015 and one wonders what has been happening since October 2014 as nobody ever replied to the problem of the previous users?
I am using Lightroom CC 2015 and although Lightroom Mobile started syncing my first collections without any problem initially, now it does not work anymore, though Mobile Sync is still in active mode, but none of the new pictures added to my desktop collections are showing on my IOS devices?
Would someone at Adobe help us with this problem... please?
Have a look on tour LR desktop if you have the good collection to sync, only one collection is sync at a time.