Unable to Auto Associate Master Contract to PO

I have created a Procurement Contract and created an Contract Purchase Agreement out of the Contract. Now, i have created a Requisition and auto created a PO out of this by associating the CPA to the Requisition.
My expectation is that the Master Contract number created initially to be auto associated to the PO created but it is not auto associating but when i am trying to do it manually i am able to auto associate.

Please let me know how to associate this automatically.