purchase order approval issue

Hi,

i have issue related to purchase order approval, below is the scenario.

C --> Buyer, B-->Manager, A--> Final Approver

1) C prepares the PO and forward for approval to B.

2) B gets the notification and he forwards for approval to A

3) A approves the PO and the status of the PO changes to approved

4) C do changes to above approved PO like line deletion / cancellation or change in qty or price, the status of PO change to required approval and revision of the PO also changing

5) When C forward same PO to B for approval the PO getting approve without getting forwarded to B  which is wrong,