Stored Procedure Parameter Problem

I have a stored procudure in SQL Server with 3 parameters:
        pa varchar(10)
        pb varchar(20)
        pc smallInt OUTPUT
and therefore I creaet a stored procedure in Delphi with parameters
        @pa     INPUT
        @pb     INPUT
        @pc     INPUT/OUTPUT
when I try to call it from Delphi, it keeps coming out an error message.
If I bind the parameters by name, it says "Parameter 'pa' not found";
if I bind the parameters by number, it says "Invalid parameter".
What can I do with this? Any advices are appriciated.