Well you try deleting all the images you have synced so far to Lightroom mobile and re-sync. You must have those Images under the collections in Lightroom.
In order to delete the synced collection from Lightroom Mobile.
Go to Lightroom Preferences << Click on Lightroom Mobile tab << Delete all Data
Once done then try to sync those images first which you need it.
Let us know if that helps.
ok, well, i didn't have to delete all the images, i just had to shut down lightroom CC completely and reopen it. then it started syncing again. haven't checked on all my collections, but it seems to have worked for the one i was having trouble with. i had gone from 189 unsorted photos down to 25 sorted and edited.
Great, thanks for sharing the solution.