I uploaded my photos to LrC and let them sync making sure to set it to sync with LrM. Then often times I review, flag and make preliminary edits using LrM since my computer has a hard time processing the power needed for editing on it. I have been doing this for years now but recently i had an album that only 25% of the flags and edits synced back to LrC. I have tried uninstalling LrC, reinstalling, clearing the cache everything short or deleting everything. I usually delete and clean up my catalog and edits on LrC after things have synced back but I have been unsuccesful at getting the info to transfer to LrC. I checked all the others to see and online Lightroom, LR CC, and LrM have the flags and edits....just not LrC.
It sounds like there was an interruption in syncing - if you check the cloud in the upper right corner is syncing paused? Sometimes there is an error that bogs down the entire sync process. Check to see if you have any sync errors in the Catalog panel - if you do delete all sync errors. You can also login at lightroom.adobe.com to see any errors there. If you do try deleting all of the sync errors from there. That may clear whatever the issue is and allow syncing to continue.