selectDocument return 0

hello,
I'm making a copy of a PDF Image to a new Document in VB .NET using the DLL library version in a Windows Service application

i'm opening the document as string (byte array) in a method

Private WithEvents _qp As PDFLibrary
.
.
.

public sub new()
  _qp = New PDFLibrary(dllPath)
  _qp.UnlockKey("XXXXXXX")
end sub

private function Func1 (doc as byte(), Optional ByVal password As String = "") as memorystream
 Dim docId As Integer
  docId = _qp.LoadFromString(inPdf, password)
  Return Func2(docId, password)

End function

private function Func2(docId as integer,password as string) as memorystream
dim ret as integer   
dim newDoc as integer = _qp.NewDocument()

ret =  _qp.SelectDocument(DocId) ' <<<<========== THIS RETURN 0 always in the service but runs correctly in a windows form application


 _
end function

What am i doing bad?

Thanks in advance
best regards

PD: i'm sorry for my english

Hello Ingo

If i execute "ret = SelectDocument(DocId)" and  "imagesFoundId = _qp.FindImages" before open the new document this runs correctly.

The problems appears when i have two documents opened.

I've debugged the service to check the path to the DLL and this is correct

Thanks in advance