capturing PS output in IPrintOemPS2::WritePrinter

I extended the oemps sample driver implementing the IPrintOemPS2 interface.
After adding the two new methods and modifying the QueryInterface to expose
also the new interface.

The method IPrintOemPS2::WritePrinter is called initially with its arguments
set to zero in order to query whether the plug-in actually supports the
method, as commented in the documentation.

If I return E_NOTIMPL from IPrintOemPS2::WritePrinter, then the pscript
driver sends the expected ps data to the spooler bypassing the
IPrintOemPS2::WritePrinter method, as expected. If I return S_OK instead,
then the method is no longer called and nothing arrives to the spooler. This
happens regardless I set the pcbWritten output argument to zero or I leave it
unchanged.

Should I do something else in addition to this to capture the pscript's
output?

Thanks in advance