When I synk a particular collection ti LR Mobile the folder appears in the online library on lightroom.adobe.com and on the app om ny Ipad. but it is empty. when I check "all photos" the photos are there but when I check the catalouges, the one in question is empty. I have been syncing collections earlier with no trubble but some collections just wont work. why is this? What have I done wrong? is there a maximum number of photos that can be in a Mobile collection?
Could you please double check if there are missing files from the photos you're trying to sync. You can also choose Library ->Find all missing photos. If so please try to wire these photos up with the original source.
If you still have the problem could you try to remove the sync.lrdata file and restart LrD. This would restart/refresh the sync (metadata only).
Removing the Sync.lrdata might fix whatever the issue is. Or, at least, might trigger it to occur again. So remove the Sync.lrdata, start Lr, and then wait until it either syncs or gets stuck again and create at this point a Lr Desktop diagnostic log which can hopefully provide some clues.
You can find the sync.lrdata file here:
On Mac it's at /Users/<user>/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Win it would be at C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata
You can trigger thediagnostic log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button. Could you please send the log - best as a private message with a downloadable dropbox link.