Active Directory replication involves a series of complex transactions. If one of these transactions fails, the problems it causes tend to be . . . well . . . complex. In general, replication problems are caused by unstable server hardware, poor network connections, and DNS errors.
The symptoms usually are a series of Error Log messages about failed replication. The logged message tells you the cause of the failure, but it doesn't necessarily tell you the cause of the problem. Several tools are available to help you get more information. They include the following:
Special diagnostic traces that put more information in the Event log
Command-line replication administration utility, REPADMIN
Graphical Replication ...