As you mentioned: credit price is empty. THis is why.
So you have to check the setting of pricing procedure and check the pricing of item w/o credit price.
If pricing looks ok (subtotal A is set and the line has value), then maybe you have to debug why vbap-cmpre is blank.
V/08: in the pricing procedure used in the sales order, subtotal "A" must be entered in a line for determining the credit value (mark the pricing procedure and double click on "Control"). Usually, the net value plus taxes is used. This way the system is determined to use this subtotal for credit pricing. The credit price is stored in field VBAP-CMPREand used for update and credit check.