Header is no longer 'sticky'

The header was able to ‘stick’ to the top of the page as I scrolled. I have checked that the option ‘sticky’ is selected in the customizer. the header changes color as i scroll but that is it, I even tried editing the header.php by deleting it all and repasting from the original downloaded header.php file. Still the same beavior.

My website is:

Try regenerating the cache from Supercache. Looks like the necessary code isn’t being outputted.