Hummingbird Minification stuck on check files

Struggling to use the minification section of this site - every time I go to this page it always shows the "check files" button - which I click and then starts a test and says "test in progress", but then after a long wait, always returns to the reduce your page load time - "check files" button screen rather than showing the results.

Other bits of the plugin work fine, but this one seems to be stuck in some kind of loop