Negative M.A.P

Hi,

Here i m facing the problem of Negative MAP.The concerned material master is maintained with two valuation type,with 1st valuation type there is Positive MAP and with 2nd there is Negavtive MAP.So when we run MB5B report for that,there is confusen.

So please suggest me with possible reasons for Negative MAP? and tell me how do i rectify this ?plz tell me with t codes.Can i use MR21 for that? or ?plz suggest.