Responsive Google Adsense ads don't work in the header ad slot.
Short Answer: Disable the header logo and header social badges (do so in the theme options >> general settings >> sticky bar & header area)
Long Answer: This is due to the way responsive Google ads detect parent element width. In Explicit's header the ad floats and thus does not have a width (the nature of CSS) and the only way to get it not to float is to disable the other floating elements within the same row - namely the logo and social badges. What further complicates this is that the logo can be a variable width so it's impossible to hard-code some sort of limit here. We even played around with absolute positioning and media query widths but could never get a responsive ad to look quite right in the header, it would always end up overlapping one of the other elements. So it's really not a great place to put a responsive google ad in the first place, thus the workaround suggested above. Of course the other option is to use a non-responsive google ad there, and as always you can use any html or standard image ad (i.e. non-javascipt) and it will be fully responsive.