I am trying to print duplex on a printer but it will not work properly.

To select the printer defined by the user I use the following code:

blnPrinterFound = False
'Find the user defined printer
For Each prnCheck In Printers
    If UCase$(prnCheck.DeviceName) =
Trim$(UCase$(aDocument(lngDocumentInfo).pddPrinter_Id)) Then
        Set Printer = prnCheck
        blnPrinterFound = True
        Exit For
    End If
Next

'If not found then set the default printer
If Not blnPrinterFound Then
    For Each prnCheck In Printers
        If strDefaultPrinterName = prnCheck.DeviceName Then
            Set Printer = prnCheck
            Exit For
        End If
    Next
End If

The printer I use is a HP4000T with duplex option.
When I do not use the above code and the default printer is the HP4000T then
I can set the duplex property to 3 (or so).
When I set the printer with the lower part of the code to the standard, ie.
it is the same HP4000T,  then changing the duplex mode will return error 380
invalid property value.

Why does it do this???