How to activate HotReload

Hot Reload doesn’t work when press Ctrl+S.

In the DEBUG CONSOLE tub, the message ‘Exception attempting to connect to the VM Service: WebSocketException: Connection to ‘http://127.0.0.1:60179/IMhpj0QQfsA=/ws#’ was not upgraded to websocket
This was attempt #200. Will retry in 0:00:01.600000.’ is displayed many times.

During running, the message ‘Flutter: Installing build\app\outputs\flutter-apk\app-debug.apk…’ is displated long time and doesn’t disappear.

When I press Ctrl+S, the message ‘Flutter: Hot Reloading…’ is displayed but the change isn’t reflected in a smart phone connected with USB.

A week ago, Hot Reload was reflected in the smart phone. I don’t know the reason why Hot Reload isn’t reflected. Could you tell me how to deal with this probrem?