.Find in this function works for the first call but will not after that

I am having an issue with this function searching for a Column header than using the column number as the reference for searching in a different sheet.

Sub Search(COL_TITLE As Variant, BOMSheet As Worksheet, prompt As String)

    Dim wb As Workbook, ws As Worksheet, found As Range
    Dim wbSearch As Workbook, wsSearch As Worksheet
    Dim rng As Range, iResultCol As Integer, iPartCol As Integer
    Set wb = ThisWorkbook
    Set ws = BOMSheet

    ' headers
    Set rng = ws.UsedRange.Rows(1)

    ' determine part number col
    Set found = rng.Find(COL_TITLE, , xlValues, xlPart)
    If found Is Nothing Then
      MsgBox "Can't find "& COL_TITLE, vbCritical, "Search failed"
      Exit Sub
    End If
    iPartCol = found.Column

    ' determine last col
    iResultCol = rng.Columns.count + rng.Column
    ws.Cells(1, iResultCol) = prompt & " Search Result"
    Debug.Print rng.Address, iPartCol, iResultCol

    Dim sFilename As String
    sFilename = Application.GetOpenFilename(Title:=prompt, FileFilter:="Excel Files (*.xls*),*xls*")
    If Len(sFilename) > 0 Then
       Set wbSearch = Application.Workbooks.Open(sFilename)
    Else
       MsgBox "No file chosen", vbExclamation
       Exit Sub
    End If

    ' find last row
    Dim iLastRow As Long, iRow As Long, sPartNo As String, count As Long
    iLastRow = ws.Cells(Rows.count, iPartCol).End(xlUp).Row
    Debug.Print "iLastRow", iLastRow

    ' search each sheet
    For Each wsSearch In wbSearch.Sheets
        For iRow = 2 To iLastRow
            sPartNo = ws.Cells(iRow, iPartCol)
            If Len(sPartNo) > 0 Then
                Set found = wsSearch.UsedRange.Find(sPartNo, , xlValues, xlWhole)
                If found Is Nothing Then
                ' not found
                Else
                    ws.Cells(iRow, iResultCol) = "Found in "& wbSearch.Name & _
                    ""& wsSearch.Name & _
                    " at "& found.Address

                    count = count + 1
                End If
            End If
        Next
    Next

    ' end
    wbSearch.Close False
    MsgBox count & " matches", vbInformation, "Finished"

End Sub

I am calling this function twice. The first time the call is:

Call Search("Part Number", BOMSheet, "Choose OCCL File")

This one works perfectly but in the second call:

Call Search("Manfacturer PN", BOMSheet, "Choose OCCL File")

The exit sub message is given, so I think the function isn't finding the column with the header "Manufacturer PN". I don't see anything wrong here and I am not getting any errors so I am kind of stuck. Code courtesy of CDP1802.

Here's the sheet with the headers for reference: Snip from Sheet