User was deleted from AD then re-added and User Synchronization Service will not sync the user
This could be because the user’s SID has changed on the back end. If everything is the same as far as name and email try this:
Basically when you switch the user from windows sync to logical it deletes the SID which we store in the db, when you switch it back the field stays empty until the sync service tries again. If that doesn’t work I will need turned up user sync logs.