Presumably Contacts, is the table occurence not shown except for the relationship line that connects to Contact_ID.
Putting one portal inside another is not supported by Filemaker. You'll need to use a different design.
Your "mini portal" is not displaying related records for a specific record in Letters, it is listing all follow up letters for all Letters sent to this contact.
thanks Phil - wasn't aware of 'nesting' portals not being supported - but on reflection now understand why - tricky to identify which related records are being referred to.
yes will rethink and redesign ....