Do you still need assistance with this? If so, can you post a sample workbook (twbx file)?
The reason might be that the secondary source does not have a matching member to the primary source.
We never really resolved this but the client decided that they could live with it.
Sharing the workbook without sharing too much info is a little tricky so I am prepared to let the matter drop.
In our case the "secondary" and primary datasources are identical, so I don't think it is the "matching member" issue you mentioned.