Why does socket.onmessage stop being called 2-3 minutes after new WebSocket()?

Question

Django

For the backend I use django-channels
js:

var socket = new WebSocket("ws://localhost:8000?session_key=...");
socket.onmessage = function() {
}

When the page is opened, onmessage works fine, but after a few minutes, messages from the client continue to arrive at the server, but messages from the server do not reach the client. Or they reach, but for some reason onmessage stops working.
What can be wrong?