Since CustomerID is, I assume, a field that uniquely identifies each record, your portal will only display one record, the same record as is the current record on you layout. If you want to display a list of all records in the portal, change the = to X so that it matches for any value. To change this, double click the relationship line.
Thanks - I tried that and I still have several portals (rather than a single portal). However now each portal row is populated with one customer details. So if I have 3 customer records "Jack", "John" and "Jill", I get 3 portals - one with all the rows as "Jack", the second with "John" and the last as "Jill".
I only need one portal - which would contain 3 rows - "Jack", "John" and "Jill".
Sounds like you have a list view layout where you can see more than one record at a time. Switch to Form view instead of list so that you see only one record at a time. If you have this relationship:
Customers::anyfield X Customers 2::anyfield
You can place a portal to Customers 2 on a layout based on Customers (Check Show Records From in layout setup) and it will list all records from that portal's table.
Thanks - that fixed it !!!