Smush server was unable to access images from your site.

I've just purchased a yearly plan for WPMU and installed WP Smush Pro, but when I try to perform a bulk smush, it gives the error "Smush server was unable to access images from your site." When I go to an individual image to smush, it says "Currently smushing" but it's been saying that for a while now. Please advise how I can make this bulk smush work. Thank you.