First take a look in your style.css file and remove or comment out the position: fixed;
around line 1177 like this –
.site-header {
background-color: #000;
left: 0;
/*position: fixed;*/
top: 0;
width: 100%;
z-index: 999;
}
The line number above are based on the default CSS. If you’ve made customizations to the style.css file then the code may appear on a different line.
Let me know if that helps.