I think the problem is in the index of the new rows but I could be

does anyone see where the problem is here?

                    //renumber the existing priorityID's in the lookup table
                    int firstIndex = RenumberPriorityID(frmAddLookupRow.StartingPriorit yID, true, frmAddLookupRow.NumberRows) + 2;

int nextPriorityID = frmAddLookupRow.StartingPriorityID;

// add the number or rows required to the lookup table
                    for (int counter = 0; counter < frmAddLookupRow.NumberRows; counter++)
     {
        TKData.TKSIMSLookupRow newLookupRow = tKData.TKSIMSLookup.NewTKSIMSLookupRow();
        newLookupRow.SiteID = txt_VesselDetail_SiteID.Text;
        newLookupRow.IMONumber = txt_VesselDetail_IMONumber.Text;
        newLookupRow.VesselName = cboVessel.Text;
        newLookupRow.ProjectName = cboProjectName.Text;
        newLookupRow.SisterGroup = cboSisterGroup.Text;
        newLookupRow.PriorityID = nextPriorityID;
        newLookupRow.IsModified = false;
        newLookupRow.ModDescription = "NA";

// tKData.Tables["TKSIMSLookup"].Rows.InsertAt(newLookupRow, firstIndex + counter);
tKData.Tables["TKSIMSLookup"].Rows.Add();

       nextPriorityID = nextPriorityID + 1;

 }
}
dgLookup.AutoGenerateColumns = false;
dgLookup.DataSource = tblLookup;
dgLookup.EndEdit();
dgLookup.Refresh();
}