Add editable column in system matrix

I have added a column in A/R invoice as,

 objForm = SBO_Application.Forms.GetForm(133, 1)
 For intI = 0 To objForm.DataSources.UserDataSources.Count - 1
                If objForm.DataSources.UserDataSources.Item(intI).UID = "AP" Then
                    Exit For
                End If
 Next
If intI = objForm.DataSources.UserDataSources.Count Then
                objForm.DataSources.UserDataSources.Add("AP", SAPbouiCOM.BoDataType.dt_LONG_TEXT, 50)
            End If
matMatrix = objForm.Items.Item("38").Specific
If matMatrix.Columns.Count = 265 Then
                matMatrix.Clear()
                objColumn = matMatrix.Columns.Add("AP", SAPbouiCOM.BoFormItemTypes.it_EDIT)
                    Else
                objColumn = matMatrix.Columns.Item("AP")
            End If
 objColumn.DataBind.SetBound(True, "", "AP")
            objColumn.Width = 65
            objColumn.Visible = True
            objColumn.Editable = True
            matMatrix.Columns.Item("AP").DisplayDesc = True
            matMatrix.Columns.Item("AP").TitleObject.Caption = "A/P Invoice"

But this column is not editable.How do convert this newly added column into editable column ?