splitting one table into two - relationships don't work

I'd like to create a simple table in a report which has 3 columns

1. Sales contract name (held in Sales Contract table)
2. Agency Primary (held in Sales Contract Agency Table, with a "primary" flagged on the row)
3. Agency Secondary (if there is any,held in Sales Contract Agency Table, with a "secondary" flagged on the row)

I have 2 data/tables: 

1. Sales Contract
2. Sales Contract Agency

The Sales Contract Agency table has a heap of agencies in it related to the Sales Contract ID. Some of the Agencies are a primary agency for the said sales contract, while others are secondary - but they are held in the same table. 

In PBI I have created one "Sales Contract Agency Primary" table, and filtered by "primary", and another table  "Sales Contract Agency Secondary" table, and filtered by "secondary" and have related both those tables back to the Sales Contract ID in the Sales Contract table. The relationships are good, so I have 3 tables:

1. Sales Contract
2. Sales Contract Agency Primary
3. Sales Contract Agency Secondary

Data from Sales Contract and EITHER ONE of "Sales Contract Agency Primary" or "Sales Contract Agency Secondary" can exist in a table, but not data from all three - I get a relationship error (and this is from the frown report)

"Cant display the data because PBI can't determine the relationship between two or more fields"

OData Error Message:
Not showing data for DataShape 'DS0' because it's not clear how these fields are related.

Any clues as to why this happens?