Syncing catalog between LR 8 on desktop and LR CC - only syncing 4000 of 24,000 photots

I recently purchased the LR CC app and was having some difficultly syncing between the online app and my ancient LR 5 catalog. So on the advice of someone here, I upgraded to the trial version of LR 8 in order to do the sync. It's started syncing finally, but it's only syncing around 4000 of 24,000 photos. How do I ascertain which 4000 it's chosen and how can I get the rest synced? I've clearly done something wrong in the early stages, but I have no idea what or how to rectify it. Thanks!