Cannot add Purchase Orders?

We have found an issue which is not happening consistently. When trying to add a Purchase Order against certain Vendors, the PO does not get added however we do not get an error either. We have identified the problem be the default setting for Payment Method.

Our vendors are all set with the default setting of check. However when we go into the PO document and change the Payment Method to Blank then the PO gets added.

Again this is not happening consistenly. Has anyone ever come across this issue. Please advise.