I had this issue at the start of the week following a problem with disk space which Just Host rectified, and then Jetpack Boost was able to regenerate critical CCS. It worked for a couple of days, and then, after I had been into an old album of images to add captions to about 30 photos it has started to give the “Failed to generate Critial CSS” message again. As a result of this my website is not displaying properly.