They should be visible in the Sync Issues folder on lightroom.adobe.com, with better information on what's the issue. Potentially you should be able to make changes that force sync on these (like a metadata change-flag or rating etc), or try to remove them.
TBH the last time things got really stuck, I did have to delete and resync 75,000 images myself-basically comments stopped coming in from client shared collections.
Sean McCormack. Author. Magazine Writer. Former Official Fuji X-Photographer.