Self hosted Matomo not tracking all visitors

Using matomo self hosted quite a while now. For additional plugins, I’ve signed up for 21 days trial of cloud matomo. My tracking code:

  <!-- Matomo -->
    <script type="text/javascript">
        var _paq = window._paq || [];
        /* tracker methods like "setCustomDimension" should be called before "trackPageView" */
            _paq.push(['setUserId', getCookie("UIDmatomo")]);
        (function() {
            var u="//";
            _paq.push(['setTrackerUrl', u+'matomo.php']);
            _paq.push(['setSiteId', '1']);

            // Add this code below within the Matomo JavaScript tracker code
            // Important: the tracker url includes the /matomo.php
            var secondaryTrackerUrl = '';
            var secondaryWebsiteId = 1;
            // Also send all of the tracking data to this other Matomo server, in website ID 2
            _paq.push(['addTracker', secondaryTrackerUrl, secondaryWebsiteId]);

            var d=document, g=d.createElement('script'), s=d.getElementsByTagName('script')[0];
            g.type='text/javascript'; g.async=true; g.defer=true; g.src=u+'matomo.js'; s.parentNode.insertBefore(g,s);

    <!-- End Matomo Code -->

Now can someone explain me this?

multiple visitor (20!!) were not tracked by my self hosted solution while it was tracked by the cloud solution. how is this possible?