I have just wanted to synchronize new folder with Lightroom on my PC. Folder consisted of 123 photos. Lightroom only synchronized 50 photos. Then I once again clicked to synchronize folder. It synchronized next 50 photos. Then it synchronized 10, 5, 5 and finally remaining 3.
I would like to synchronize whole folders instead of doing it partialy.
Please check the sync activity in Lightroom Desktop Preferences>Lightroom mobile.
This way you can specify the images that are not syncing and then delete or move them.
See Problem syncing photos with Lightroom mobile for more info.