FCM notifications not being displayed by devices - php

Question

I have a PHP application, through which users can send out alerts (to read a message) to clients who have an app installed. It seems to work pretty well, but I have a consistent small percentage of recipients who say they never get any alerts on their phones. The problem doesn't seem to be restricted to any particular model of phone, and the few users I've spoken to have had the relevant notification permissions allowed.
My code:
First I loop through the recipients identified by the web application and create a notification packet per recipient device:
$packets = [];
foreach($messages_to_send as $msg)
{
$message_id = intval($msg['message_id']);
$device_token = $msg['device_token'];
if(!isset($packets[$device_token]))
$packets[$device_token] = [];
if(!isset($packets[$device_token][$message_id])) {
$packets[$device_token][$message_id] = [
'notification' => [
'title' => 'A new message has been received!',
'body' => 'Open notifications',
'sound' => 'default',
'badge' => '1'
],
'priority' => 'high',
'data' => [
'type' => 'sms',
'msg_id' => $message_id,
]
];
}
}
Then I loop through those packets and send a cURL request to FCM per message, per device:
$headers = [];
$headers[] = 'Content-Type: application/json';
$headers[] = 'Authorization: key='. $server_key;
foreach($packets as $device_token => $messages)
{
foreach($messages as $msg_id => $messages_to_send)
{
$msg_json = json_encode($messages_to_send);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $fcm_url);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST,"POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $msg_json);
curl_setopt($ch, CURLOPT_HTTPHEADER,$headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
//Send the request
$curl_response = curl_exec($ch);
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
$curl_error = null;
if( ($httpCode != 200) || (curl_errno($ch)) )
$curl_error = ($httpCode != 200) ? 'Firebase response not received (200)' : 'Firebase cURL error: ' . curl_strerror(curl_errno($ch));
curl_close($ch);
$curl_response = json_decode($curl_response, true);
$status = null;
$sent_ok = 0;
if($curl_error !== null)
$status = $curl_error;
else
{
if($curl_response['success'] != 1)
$status = 'FCM failure';
else
{
if(!isset($curl_response['results'][0]['message_id']))
$status = 'Device token no longer recognised by FCM';
else
{
$sent_ok = 1;
$status = $curl_response['results'][0]['message_id'];
}
}
}
// Register the sending status per recipient
}
}
I would love to get this issue closed off. Is there anything wrong with the way I am sending out these notifications? If not, what (other than express permissions) would cause a device to not display an alert?