Creating change orders in Blanket Purchase agreement is not taking Approval rule

Say in a BPA, we already have the start date and end date in the document, say we have the same expiration date in the line, and next when we create a change order by changing the amount and saving it, it is not taking the approval rule, then when we remove the expiration date from the line and save it, it is taking the approval rule. Once this is submitted and change order is approved, next time when we again change amount and save, it is not again taking approval rule. Again if we put back the empty expiration