Could you find the Collections you are syncing here Adobe Photoshop Lightroom (Sign in with your Adobe Id)
Did you Orignally Imported these Pictures from your Mobile or computer?
Could you make sure that Pictures Missing in Collections are actually not Missing from Lightroom Catalog(on your Computer).
Missing Pictures will show you " ! " sign on top right of image Thumbnail.
Other way to check all Missing Pictures in your catalog :-In Library Module>>Library(from menu bar)>>"Find all missing Photos"
Also which version of Lightroom and what Operating System does your Computer have?
the pictures are not showing as missing. i can open up any of the pictures in these collections fine-even open into photoshop which couldn't happen if they are missing.
The pictures are on my desktop, i placed them in a collecton and have the sync on for these collections just the same as i do for the collections that are syncing. in my mobile devicies ( iphone and ipad) they are showing as a collection with the name but the albums are empty.
I stand corrected:(-they were there before but i just checked again and tried to open one in photoshop and they are obviously missing-i guess i have a problem there.I will go find them. thank you for your help.