I am having the same problem.
I have left the computer running for many days with Sync running and very very few files seem to be making their way into storage on the Cloud and then as viewable images on my devices.
Tiffs and JPGs seem to be slightly better but Lr seems to have a problem with .NEFs even though I created Smart Previews for them.
Another problem is that when you do open a Mobile collection, your screen is filled with empty frames (image n/a) and you have to wade through all that log-jam to find the few images which have actually come through!
Suggestion for Adobe: Please change Lr Sync so that no empty frames display on our Devices but ONLY the frames which actually contain viewable images!
Lr Mobile is a brilliant concept: I just hope that Adobe can eventually get it working because at the moment it is not very useful!
Could you both send me a LR Desktop diagnostig log - best as a private message with a downloadable dropbox link.
You can trigger the log via LR Desktop preferences -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button.
Dropbox Link sent via PM.
Let me know if you have any problems downloading it.
Meanwhile it is almost impossible to re-edit any files which I get back from Lr Mobile.
The Develop Sliders for Exposure, Contrast & HL open (after a very long wait!) but none of the Develop Sliders below HL ever become operative.