Could you find the Collections you are syncing here Adobe Photoshop Lightroom (Sign in with your Adobe Id)
Also ,
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.
If so Adobe Photoshop Lightroom Help | Locate missing photos
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?
Regards
~Akash