Using fields from a subreport in the formula editor.

I'm using crystal 9. I have a subreport within a main report. The main report is invoices for customers. The subreport is in a footer and it contains payment information. It is linked to the main report by the Invoice's Packslip ID. The subreport uses a separate stored procedure to find all the payments associated to that single invoice. The subreport works great but I need to supress some of the payment records if they are not credit cards (payment type). The payment type is a field in the subreport but when I try to create a formula to supress records the only fields I have available are fields that are either in the main report or fields from the Main stored procedure. Why don't I have fields pulled in from the subreport's stored procedure available for the editor?