What is the most likely cause of replication issues in a domain?

The most likely cause of replication issues in a domain is that the infrastructure master and global catalog are on the same domain controller (DC). This situation can lead to problems because the infrastructure master needs to be on a different DC from the global catalog to function correctly in a multi-domain environment.

When the infrastructure master and global catalog reside on the same server, the infrastructure master may not update the objects in other domains appropriately when there are changes. This is primarily due to the fact that the global catalog is already aware of the objects in its own domain, which can prevent proper updates from occurring if the infrastructure master does not detect those changes.

To ensure smooth replication, it's important to maintain distinctive roles for these services on separate DCs, as this facilitates proper communication and updates across different domains within the Active Directory environment.