When synchronising you may see the message
"The following child diagrams of referenced parent diagrams are missing in this model"
This indicates that a reference diagram has child pages that are not present in the area model, and offers to import them.
If you import them only those pages that were listed are imported. If they themselves have missing child pages then they will not be imported. You can synchronise each page one at a time from the Page Details to add them, of or Synchronise All, but this is slow.
The best way to add a reference model page is to use the File Manager
From the area model, go to File Manager, then select Last Ref. Then import the diagrams you want to add. Select Child Pages to ensure that the entire child hierachary is also imported
When importing from a reference model the diagrams are added as referenced diagrams.