Websocket - Why does not every client receive the message?

Hello!

I have implemented a websocket server using this tutorial from AWS. I use the websocket to send real-time messages between my Unity project (simple desktop app) and a website.

Example:

When I send the following message then a button on the website should be clickable for everyone.

{"action": "sendmessage", "message": "button"}

However, out of the 200 connected clients, only 20 are actually receiving the message and able to click the button.

Sending a message in C#:

void SendMessage()
    {
        var msg = new 
        {
            action = "sendmessage",
            message = "button"
        };
        _ws.Send(JsonConvert.SerializeObject(msg));
    }

Receiving messages in Javascript:

const socket = new WebSocket("wss://xyz.execute-api.eu-central-1.amazonaws.com/x");
socket.onmessage = (event) => {
  if (event.data == "button") 
  {
    button.disabled = false;
  }

I followed all the settings in the AWS console as per the tutorial linked above. Do you have any idea what should be the problem?