To Track those issues we are getting a cool utility with SharePoint 2013.
It is miisclient located in /15/Synchronization Service/UIShell
After Open this utility we can see all synchronization attempts and the results.
It is called "Synchronization Service Manager on SERVERNAME".
There we can identify all attempts, time, status and Errors.
Here it says, "Replication Access was Denied" when trying to sync.