Google Invisible reCaptcha not stopping SPAM submits - php

Question

I have implemented Google reCaptcha on a site, for the newsletter subscription form, and it does not stop spam. I got up to 20 spam subscribers per day!
What am I not doing correct? Please have a look at the code:
HTML
<form method="post" action="databasepage.php" id="formid" >
<div id="g-recaptcha-answer"></div>
<input name="email" placeholder="Email..." type="email" required>
<button class="g-recaptcha" data-sitekey="my_key" data-callback='onReturnCallback'>Submit</button>
</form>
jQuery
<script>
var onReturnCallback = function(response) {
document.getElementById('g-recaptcha-answer').innerHTML = '';
var url='proxy.php?url=' + 'https://www.google.com/recaptcha/api/siteverify';
$.ajax({ 'url' : url,
dataType: 'json',
data: { response: response },
success: function( data ) {
var res = data.success.toString();
if (res == 'true') {
document.getElementById('g-recaptcha-answer').innerHTML = 'Please wait for a redirect.';
document.getElementById("formid").submit();
}
else {
document.getElementById('g-recaptcha-answer').innerHTML = 'Verification incorrect.';
grecaptcha.reset();
}
}
});
};
</script>
proxy.php
<?php
if (!isset($_SERVER['HTTP_X_REQUESTED_WITH'])) {
header('HTTP/1.0 403 Forbidden');
die('You are not allowed to access this file.');
}
header('Content-type: application/json');
$url=$_GET['url'];
$response=$_GET['response'];
$secret = "secret_key";
$params = array('secret'=> $secret, 'response'=> $response);
$json=file_get_contents( $url . '?secret=' . $secret . '&response=' . $response);
echo $json;
?>
I have tested this with Incognito Mode, I get the step when I have to verify pictures, if I am not logged in Gmail etc., in reCaptcha admin area I have no errors, but I STILL RECEIVE SPAM EMAILS IN MY DATABASE!!!
What is wrong in my approach?