DataTable Fixed Header Not Being Applied

Your fiddle seems to be missing the dataTables.fixedHeader plugin as well as the fixedHeader.dataTables CSS file which was causing you the page from rendering properly (with the fixed headers).

Working DEMO: - https://jsfiddle.net/s2xbf9d0/7/

Hope this helps!

For the fixedHeader plugin to work you also need the css file:

<link rel="stylesheet" type="text/css" href="https://cdn.datatables.net/fixedheader/3.1.2/css/fixedHeader.dataTables.min.css">

Check this update:
https://jsfiddle.net/s2xbf9d0/6/