Can not delete item in Putchase order for services

Hi Experts,

our system is upgrade from 4.7 to 6.0  and when we create purchase order for services with Account assignment Cat. - F/order/ and Item cat. D/service/ and when we put service in Item detail save PO. With T-Code ME22N delete in Item detail one or more item and save PO. But when we open with ME22N PO the items is not delete , only delete G/L account and Order. In print form we print delete item.I can't understand what's happened.