Styjun

My Bootstrap Navigation Bar is supposed to be sticky, but it isn't. (I'm using .sticky-top)
I read a few posts about this problem, but i don't understand how and why this is happening. (I'm not very good at bootstrap / CSS).
This is my navbar code:

<header>
 <img class="col-md-12 d-none d-md-block" src="assets/banner website.png" alt="banner" width="1900">
 <img class="col-sm-12 d-mone d-block d-md-none img-fluid" src="assets/banner website tel.png" alt="banner4phones" widht="100%">
 <nav id="navbar" style="overflow: hidden;" class="navbar navbar-expand-md bg-dark navbar-dark">
 <a class="navbar-brand" href="#"><img src="assets/Het logo der logo's voor nav.png" alt="logo" width="40px;"></a>
 <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#collapsibleNavbar">
 <span class="navbar-toggler-icon"></span>
 </button>
 <div style="position: relative; z-index: 3000" class="collapse navbar-collapse" id="collapsibleNavbar">
 <ul class="navbar-nav mr-auto">
 <li class="nav-item fade-in <?php echo ($page == 'home' ? 'active' : ''); ?>"><a href="<?php echo ($page == 'home' ? 'javascript:void(0)' : 'https://jinvantongeren.nl'); ?>" class=" nav-link">Hoofdpagina</a></li>
 <li class="nav-item fade-in <?php if($page=='contact')echo 'active';?>"><a href="<?php echo ($page == 'contact' ? 'javascript:void(0)' : 'https://jinvantongeren.nl/contact.php'); ?>" class="nav-link">Contact</a></li>
 <li class="nav-item fade-in <?php if($page=='over')echo 'active';?>"><a href="<?php echo ($page == 'over' ? 'javascript:void(0)' : 'https://jinvantongeren.nl/over'); ?>" class="nav-link">Over</a></li>
 <li class="nav-item dropdown"><a class="nav-link dropdown-toggle" href="javascript:void(0)" id="navbardrop" data-toggle="dropdown">Vakken</a>
 <div style="position: absolute; z-index: 1000;" class="overflow-auto dropdown-menu">
 <a class="dropdown-item <?php if($page=='Biology')echo 'active';?>" href="<?php echo ($page == 'Biology' ? 'javascript:void(0)' : 'https://jinvantongeren.nl/Biology'); ?>">Biology</a>
 <a class="dropdown-item <?php if($page=='Assignment')echo 'active';?>" href="<?php echo ($page == 'Assignment' ? 'javascript:void(0)' : 'https://jinvantongeren.nl/assignment'); ?>">English</a>
 <a class="dropdown-item <?php if($page=='ICT en Design')echo 'active';?>" href="<?php echo ($page == 'ICT en Design' ? 'javascript:void(0)' : 'https://jinvantongeren.nl/Website opdracht/index.html'); ?>">ICT en Design</a>
 </div>
 </li>
 </ul>
 <ul class="navbar-nav ml-auto">
 <?php if(isset($_SESSION['User'])) 
 if($statusImg == 1) ?>
 <img src="assets/profile/profileimg <?php echo $userName ?>.<?php echo $fileactualext ?>" class="img-fluid rounded-circle navbar-brand" style="width: 40px; height: 40px;" alt="profielfoto" />
 <?php ?>
 <?php if(isset($_SESSION['User'])) ?>
 <li class="nav-item <?php if($page=='control')echo 'active';?>">
 <?php if(in_array('Admin', $explodeGroep)) ?>
 <a href="https://jinvantongeren.nl/control?groep=<?php echo htmlspecialchars(join(', ', $explodeGroep)); ?>&uid=<?php echo $userName ?>" class="nav-link">Beheer</a> 
 </li> <?php ?> <?php if(isset($_SESSION['User'])) ?>
 <li class="nav-item <?php if($page=='exclusive')echo 'active';?>">
 <a href="https://jinvantongeren.nl/exclusive?groep=<?php echo htmlspecialchars(join(', ', $explodeGroep)); ?>&uid=<?php echo $userName ?>" class="nav-link">Mijn content</a> 
 </li><?php ?>
 <?php if(isset($_SESSION['User'])) ?>
 <li class="nav-item <?php if($page=='Personal')echo 'active';?>">
 <a href="https://jinvantongeren.nl/personal?uid=<?php echo $_SESSION['User']; ?>" class="nav-link">Mijn account</a> 
 </li> 
 <?php ?>
 <li class="nav-item">
 <?php if(isset($_SESSION['User'])) ?>
 <form class="form-inline" method="post" action="includes/logout.inc.php">
 <button class="btn-primary btn" type="submit" name="Logout">Uitloggen</button>
 </form> 
 <?php ?>
 </li>
 <?php if(!isset($_SESSION['User'])) ?>
 <li class="nav-item <?php if($page=='Login')echo 'active';?>">
 <a href="https://jinvantongeren.nl/login" class="nav-link">Inloggen</a>
 </li>
 <?php ?>
 </ul>
 </div>
 </nav>
</header>

I've tried solving this, using the newest version of bootstrap (4.3.1), but that didn't work.

Does anyone knows a solution?
With kind regards,

Have you tried adding it to the header <header class="fixed-top"> or just add position fixed to your element? You may be overwriting the css somewhere, but it's hard to tell. If you could provide a link or more info it may help. Hope this helps.