After posting credit memo when checked debit note invoice value & quantity is picking wrong

Hi Guys,

    When i post my credit memo and then when i check in debit note i am getting incorrect Invoice quantity and Value. This problem comes when i post credit memo of Invoice posted for single purchase order with two or more line items.

But when i post credit memo for invoice posted for multiple PO's the Invoice quantity and value in debit note is coming right.

I want to know in what way those conditions are picked from respective table. Why its coming right for Invoice with multiple PO's and why not its coming right for Invoice with single PO with multiple line items. What changes i have to make for getting it right.

Thanks

Prasan