C# ServerReport Render using rs:PersistStreams and rs:GetNextStream on WinForms

I'm developing a Winforms desktop Application and I'm using rs:PersistsStreams and GetNextStream to render my ServerReport to Image, but it is not always successful, for the same parameter, sometimes I could get only 1 page, but the other times i get 2 pages. Below the code i use to render the report:

NameValueCollection firstPageParameters = new NameValueCollection();
        firstPageParameters.Add("rs:PersistStreams", "True");

        // GetNextStream returns the next page in the sequence from the background process
        // started by PersistStreams.
        NameValueCollection nonFirstPageParameters = new NameValueCollection();
        nonFirstPageParameters.Add("rs:GetNextStream", "True");
        m_streams = new List<Stream>();
        string mimeType, filenameExtension;

        Stream pageStream = report.Render("Image", deviceInfo, firstPageParameters, out mimeType, out filenameExtension);
        while(pageStream.Length > 0)
        {
            m_streams.Add(pageStream);
            pageStream = report.Render("Image", deviceInfo, nonFirstPageParameters, out mimeType, out filenameExtension);
        }

My question, why it is not getting the same result although using the same parameter? is there any way that I could use to check render status when using rs:GetNextStream?

Thanks and Best Regards Regan