Nodejs websocket server

I found a simple websocket server in nodejs.

var WebSocketServer = require('ws').Server,
  wss = new WebSocketServer({
    port: 8080
  });

var clientsCount = 0;

console.log('clients,rss,heapTotal,heapUsed');

setInterval(function() {
  console.log('%d,%d,%d,%d', clientsCount,
    process.memoryUsage().rss,
    process.memoryUsage().heapTotal,
    process.memoryUsage().heapUsed);
}, 10000);

wss.on('connection', function(ws) {
  clientsCount++;
  console.log(clientsCount);

  ws.on('message', function(message) {
    console.log(message);
  });

  ws.send('something');

  ws.on('close', function() {
    clientsCount--;
  })
});

The script works fine but sometimes when a client connects and sends a data, the server doesn't accept the message; ws.on('message', function(message){...}); doesn't seem to trigger when the event occurs.