i have a problem on a report

i have a report "VENDOR PAYMENTS"----data source---vendorpayqry

i have a table "VENDORPAYADVANCE"--fields-- "AMOUNT"-- "PONUMBER"

IN the report footer i have a field called :ADVANCE PAID
i get the value form this....=IIf(IsNull([PONUMBER]),0,DSum("[AMOUNT]","VENDORPAYADVANCE","[PONUMBER]=" & [PONUMBER]))

IT GIVES THE VALUE CORRECTLY IF ALL RECORDS ARE DISPLAYED.
BUT IF I SELECT RECORDS USING DATE RANGE AND IN THE DATE RANGE IF MORE THAN ONE PONUMBER IS THERE THEN ---" THE ADVANCE AMOUNT OF THE FIRST PO ONLY IS DISPLAYED"
Please help