Data truncated when posting form data from server side

I am posting data to an external website server side. This works when the data parameter is less than about 640 characters. The data gets truncated beyond around 640 characters. Can anyone tell me why this is and how to fix it? Any help would be greatly appreciated. The code I'm using is below:

Function PostData(TransId As String, QuoteNumber As String, data As String) As String
        
        Dim url As String = "http://localhost:49608/test2.aspx"

        Dim encoding As New ASCIIEncoding()

        Dim request As WebRequest = WebRequest.Create(url)
        request.Method = "POST"
        Dim postData As String = "WAA_PACKAGE=Vendor&WAA_FORM=blahblaj&WAA_HTML3CLASS=nHTML&VendorID=ENG3135&"

        postData += "TransID=" + TransId + "&"
        postData += "QuoteNo=" + QuoteNumber + "&"
        postData += "Data=" + data
        Dim byteArray As Byte() = encoding.GetBytes(postData)

        request.ContentType = "application/x-www-form-urlencoded; charset=UTF-8"

        request.ContentLength = byteArray.Length

        Dim dataStream As Stream = request.GetRequestStream()

        dataStream.Write(byteArray, 0, byteArray.Length)

        dataStream.Close()

        Dim response1 As WebResponse = request.GetResponse()

       
        dataStream = response1.GetResponseStream()

        Dim reader As New StreamReader(dataStream)

        Dim responseFromServer As String = reader.ReadToEnd()

        
        reader.Close()
        dataStream.Close()
        response1.Close()


        Return responseFromServer.Trim().Replace(vbCrLf, "")
    End Function