ZMQ, Autobahn, Ratchet -> Sometimes not pushing messages - symfony

Question

I'm facing quite an issue and I really don't know what it's caused by at all. A lot of stuff on my website gets done via websockets, for example pushing messages to clients.
For pushing I'm using ZMQ and Ratchet.
This is the php code:
$UserMessage = array(
'user' => $userid,
'message' => 'Search started',
);
$context = new \ZMQContext();
$socket = $context->getSocket(\ZMQ::SOCKET_PUSH, 'my pusher');
$socket->connect("tcp://localhost:5555");
$socket->send(json_encode($UserMessage));
This is the push-server:
<?php
require dirname(__DIR__) . '/vendor/autoload.php';
$loop = React\EventLoop\Factory::create();
$pusher = new Some\Bundle\Topic\Pusher();
// Listen for the web server to make a ZeroMQ push after an ajax request
$context = new React\ZMQ\Context($loop);
$pull = $context->getSocket(ZMQ::SOCKET_PULL);
$pull->bind('tcp://127.0.0.1:5555'); // Binding to 127.0.0.1 means the only client that can connect is itself
$pull->on('message', array($pusher, 'onMessage'));
// Set up our WebSocket server for clients wanting real-time updates
$webSock = new React\Socket\Server($loop);
$webSock->listen(8181, '0.0.0.0'); // Binding to 0.0.0.0 means remotes can connect
$webServer = new Ratchet\Server\IoServer(
new Ratchet\Http\HttpServer(
new Ratchet\WebSocket\WsServer(
new Ratchet\Wamp\WampServer(
$pusher
)
)
),
$webSock
);
$loop->run();
The problem is: sometimes messages don't get pushed AT ALL. This is happening very randomly. Sometimes it works like a charm for 20 messages in a row, sometimes not.
Does anyone have any idea what this might be caused by? I'm using nginx as a webserver by the way. Are there any kind of logs that might help out?
Regards