Error at the time of billing due list

Dear SD gurus,

I have created a sales order with 1 line item and order quantity as 20.With reference to this order I have created two deliveries each of 10 qty.I am trying to invoice both the deliveries by using VF04(Billing due list), I am able to see only First delivery document.The second delivery delivery is not displaying in billing due list.What could be the reason.

Please guide me.

Regards,

Hari Challa.