I would recommend creating a Windows Event Log Monitor for the following Windows events that are written to the event log of the domain controller when replication fails.
- Net Logon Event ID 5805
- NTDS Event ID 1083
- NTDS Event ID 1265
- NTDS Event ID 1311
- NTDS Event ID 1388
- NTDS Event ID 1645
- SceCli event ID 1202