Domain Controller Replication Error

I have 2 domain controllers, DC01 and DC02. When I try to force replication between them, I get the following error:

Replicate Now

The following error occurred during the attempt to synchronize naming context “romegagov.org” from domain controller DC01 to domain controller DC02: The source server is currently rejecting replication request.

This operation will not continue.

Can anyone please tell me what is causing this problem? Thanks.