Hello George,
Due date calculation for Credit memo's will be as follows.
1. Credit memo will pick in the payment proposal, if we execute payment on or after due date of credit memo document.
2. If you would like to pick the credit memo same as normal accounting document, then you need to update the relevant accounting document number details in the invoice reference field in the credit memo document.
3.If you would like to pick the credit memo immediately as and when posted, then you need to update the invoice reference field with indicator "v"
4. If the invoice reference field is blank in credit memo document, then this credit memo will pick into the payment proposal only after due date /base line date maintained in the credit memo document.
5. If the invoice reference field is blank in credit memo document, then it won't be pick up in payment proposal even though you enter the next run date is greater than the due date of credit memo document.
6. If you are executing payment proposal prior to due date of credit memo, if won't pick up in the payment proposal if the invoice reference field is blank. It won't pick in this payment proposal even though if you enter next run date is greater than the due date of credit memo document.
You can also refer sap note : 22856 regarding due date calculation in case of credit memo.
Hence you may verify the dates/ indicator in invoice reference field in the credit memo.
Hope the above clarification will clarify your issue/
Thanks
Prasad