Fixed header not working (I've looked at so many posts, sorry)

I’ve done the “[table id=123 datatables_tabletools_fixedheader=”true” /]” with my table id and the extension installed, no go.

arstechnologies.com/case-studies is the page I’m having trouble with (among others). I don’t have any rowspans or colspans and the header still wont stay fixed.

https://wordpress.org/plugins/tablepress/