While syncing images in Lightroom, one or more images remain in syncing state forever. Every time you launch Lightroom, you see that the same number of images are syncing and you do not know which images are stuck in syncing.
Solution
Solution 1: Filter the images based on sync status
Click the filter icon at the top and select Sync Status > Syncing.
Lightroom now displays those images that are trying to sync.
Select all the images and do one of the following:
Delete the selected images and then import them again in Lightroom.
Apply a flag or star rating to the selected images.
Solution 2: Check the Deleted folders
If filtering the images based on sync status returns no result, then chances are that the photo which is stuck in syncing state is in the Deleted folder.
Click the My Photos icon in the left panel and then select Deleted folder.
In the Deleted folders, look for images with the little blue syncing icon. Restore these images.
Once the images are synced successfully, you can delete the images again.