This HTML works as expected here:
<!DOCTYPE html>
<html>
<head>
<title>Test</title>
<style type="text/css">
.page-break {
display:block;
clear:both;
page-break-after:always;
}
</style>
</head>
<body>
<p>On first page.</h1>
<div class="page-break"></div>
<p>On second page.</h1>
</body>
</html>
Maybe other CSS selectors in your document prevent page breaking?
If you can post your complete document containing and all related resources it would be easier to investigate this.