Problem with Approval Procedure--After got approved user can't edit the document at any condition

hi

I'm Getting Problem With Approval Procedure..

problem : End user post a  Documents(PO,GRPO,AP INVOICE..)

1st it will goes to Manager and HOD, after got Approved document, user can able to add the document but the user is modified the Document then simply added (@ the time approval  procedure is not raised )

After got approved user can't edit the document  at any condition ,if he want edit again it will start the  approval...

Please help me in this case...

Thanks&regards

Andakondaramudu