The "Last updated" column in the PO module:
only shows when the last order update has come in from a specific vessel. This will turn red after no update within 7 days.
If you have any red dates, this is not an indication that the replication is not working, it just means that there has not been an order update from the vessel for the last 7 days.
However, if you still suspect that this might be caused by replication issues, please visit this article for more info on what to do if you suspect replication issues.