With standard configuration, system would fetch the payment terms as follows:-
- If you generate a document with reference to another document, system would fetch from the preceding document
- If the Payment terms (due to some reason, which should not be) is blank in the preceding document, then it would copy from Payer
In your case, if both the above are not happening, then definitely, some exit is causing this issue which you need to check in debug with different customer / document type
G. Lakshmipathi