If there are any sync issues, click on them to know more, find the original for those images, and back them up safely before you empty the Sync issues from Lightroom Web. As long as there are Sync issues on the web, you might encounter trouble with syncing images and edits.
In the Lightroom mobile app, tap on a photo that is missing when viewed with the link. After selecting an image tap on the cloud icon. This should force the image to start uploading. When the sync is complete you will see a checkmark on the cloud. You may need to do this for all of the missing images.