Custom Joined datasource cannot be exposed

Hello,

We have created a joined data source combining C4C standard data sources:

* Opportunity header and item

* Contact master data

* Account master data

We want to expose this joined data source to BW.

But the Expose button is greyed out.

(We are able to expose standard datasource such as Contact master data individually though).

Can anyone advise what could be the issue? Can a custom joined data source cannot be exposed to BW?!

Thanks