Error in raising forest functional level?

I got the error message when try to raise forest functional level to prepare for installing exchange server 2010 on Windows Server 2008 R2.

The functional level could not be raised. The error is: The FSMO role ownership could not be verified because its directory partition has not replicated successfully with at least one replication partner.

After a research on google, I know it's because of wrong replication between two DCs. How can I solve the problem?

Thanks.