how are you loading the javascript?
Are you per chance loading it in the <head>? Then probably you forgot to add the 'defer' attribute so it runs after the page has loaded
<head>
<script src="myfile.js" defer></script>
</head>
<head>
<script src="myfile.js" defer></script>
</head>
if this doesnt solve it, check if you browser console shows errors