That's more then a broad, if not very vague question. Do you really expect a specific answer? One reason: Bad network infrastructure.
Reasons for replication latency over different data center and data domains
Issue is that on publisher, in replication monitor we are seeing replication is in sync, but our own monitoring which runs on the subscriber, reports that the column TIME in the table MSreplication_subscription is not updated and shows old value. How is this possible that on publisher replication is in sync and on subscriber it is not? What are some of the reasons for replication latency when working with different domains while same domain doesn't have any issue? Is there specific sp or commands or anything that we get alerts for latency issue happens and how to overcome this latency issues.