cannot update record from MSHFlexgrid..help!!!

[Apek]

i want to update certain records from my mshflexgrid.
but i get the error "field cannot be updated".
Why??

this is my code to update:

Dim i
With mshRejLot
For i = 1 To rs.RecordCount
    .TextMatrix(i, 0) = i
'mshRejLot.TextMatrix(i, 4) = Format(rs!amount, "dd/MM/yyyy")
    .TextMatrix(i, 7) = Round(rs!amount, 2)
    .TextMatrix(i, 8) = Round(rs!Origin_Cost, 2)
    'rs.Find "rs!JOB_NO = '" & .TextMatrix(i, 2) & "'"
    rs("AMOUNT") = Round(.TextMatrix(i, 7), 2)
    rs("ORIGIN_COST") = Format$(Round(.TextMatrix(i, 8), 2), "Currency")
    rs.Update
    If .TextMatrix(i, 7) = "" Then
        .Row = i
        .Col = 7
        .CellBackColor = vbRed
        .Row = i
        .Col = 8
        .CellBackColor = vbRed
    End If
    Dim k
    'For k = 1 To rs.RecordCount
    'rs.Find "rs!JOB_NO = '" & .TextMatrix(i, 2) & "'"
 
rs.MoveNext
    'Next k
Next i
End With

and this is my sql statement:

rs.Open "SELECT REJECT_LOT.PART_CODE, REJECT_LOT.JOB_NO, REJECT_LOT.SCRAP," & _
        "REJECT_LOT.TRANS_DATE, REJECT_LOT.REASON,REJECT_LOT.DETAIL," & _
        "(([REJECT_LOT].[SCRAP]/1000)*[Cost_Price].[Origin_Cost]) AS AMOUNT," & _
        "Cost_Price.Origin_Cost FROM Cost_Price " & _
        "RIGHT JOIN REJECT_LOT ON Cost_Price.Part_Code = REJECT_LOT.PART_CODE WHERE" & _
        "(((REJECT_LOT.TRANS_DATE)BETWEEN #" & dari & "# And #" & hingga & "#)" & _
        "AND ((REJECT_LOT.REASON)='DDI'))", conn, adOpenKeyset, adLockOptimistic

please HELP!!!!