Mavlink sending message and didn't response message (c#)

I’m Used this " "
and I use “Silicon Labs CP210x USB to UART Bridge (COM6)”.

step1. connect Serialport - success
In pixhawk2.1, the default information is displayed. (GPS, etc.)

public MavLinkSerialPortTransport mavLinkSerialPortTransport = null;
public Ardupilot()
mavLinkSerialPortTransport = new MavLinkSerialPortTransport();
mavLinkSerialPortTransport.SerialPortName = SerialPortName;
mavLinkSerialPortTransport.BaudRate = BaudRate;
mavLinkSerialPortTransport.OnPacketReceived += new PacketReceivedDelegate(recvPacket);

private void recvPacket(object sender, MavLinkNet.MavLinkPacket packet)

step2. Test Sending Message - sucess
But the delegate does not respond. :sob:

public void MissionCount()
UasMissionCount uasMissionCount = new UasMissionCount()
MissionType = MavMissionType.Mission,
TargetComponent = 1,
TargetSystem = 1,

I sent a lot of other messages, but not all of them answered.