DataGrid.RowDetailsTemplate

Hello

I'm trying to see the details of the invoices when I select the line of one of them.
For this I created a datagrid with invoices.
After I added a DataGrid.rowdetailstemplate where i define the fields i want to appear when I select an invoice.
When I select one of them with the header line appears but the invoice lines do not appear.

I've tried several ways and I can't get them to appear.
I don't know exactly when and how you have to load the information of the details.

Can someone help me?
Thank you.