Puede que sea el Encoding. Échale un ojo a:
Código (vbnet,2,13) [Seleccionar]
Private Function UnZip(ByVal compressedText As String) As String
Dim gzBuffer As Byte() = System.Text.Encoding.ASCII.GetBytes(compressedText) 'Convert.FromBase64String(compressedText)
Using ms As New MemoryStream()
Dim msgLength As Integer = BitConverter.ToInt32(gzBuffer, 0)
ms.Write(gzBuffer, 4, gzBuffer.Length - 4)
Dim buffer As Byte() = New Byte(msgLength - 1) {}
ms.Position = 0
Using zip As New GZipStream(ms, CompressionMode.Decompress)
zip.Read(buffer, 0, buffer.Length)
End Using
Return System.Text.Encoding.UTF8.GetString(buffer)
End Using
End Function