I have a shared album that has about 250 photos in it. WHen I check Lightroom Desktop and Lightroom Web, all of the photos are there. When I look in Lightroom Mobile only 38 of the photos are there. I tried making a new album with all of the same photos and the same 38 photos show up in mobile.
It is possible that these the photos not showing were deleted at one point and restored, although I am not positive if that is true for all of these photos.
I have also tried to sign out of mobile and sign back in.
When viewing your own account (signed in as the owner), you should always see all your photos -- even if they were deleted and restored.
Though you didn't say you were signed into another account, what this reminds me of is how photos you have applied the "rejected" flag on will not show to other users who are viewing your shared album. Is this possibly what's happening?
While signed in as the owner of your shared album, look to see if any of the photos have "rejected" applied -- those will be hidden from view of other users (guests) viewing your shared album. If "rejected" is applied, turn that off... Do you then see the photos you expect to see on the other devices?