Cannot copy receipt into a voucher

 Hi.  We have a PO that was created for 2 lines;  line 1 with qty 4 and line 2 with qty 5.  A receipt was also created for these.  We have paid 2 invoices on 12/1, the first against line 1 for a qty of 1 and the other for line 2 with a qty of 1.  This means we have remaining of 3 on line 1 and 4 on line 2.  We have another invoice to pay but when the voucher is created, we cannot copy the receipt into it as it does not show up to pick.  Why?