Firstly, it's always helpful to tell us the exact version number of the products you are using. "Latest version" often proves to be incorrect, so iyt's better to provide the version number.
Secondly, LrMobile does not sync directly to LrClassic (though in our minds we often think of it that way). Basically changes made in any one of our client apps (Classic, Mobile, Lightroom desktop, etc.) sync to our accounts on the Adobe cloud servers, and from there the changes will sync to all of our other connected apps. So when syncing in one app stops, as in your case, the first thing to check is where in the chain is it failing, and to do that we need to look directly into our cloud account (using the LrWeb app) to see if the specific changes made in one client appear there, i.e. in you case are the changes made in LrMobile visible in the cloud? If not, the problem lies between Mobile and the cloud....if yes, the problem lies between the cloud and Classic.
So that's the starting point....to use the LrWeb app, login to your account in any browser window at lightroom.adobe.com and check for the missing changes, then let us know what you find (and confirm the version numbers).