[WP Smush Pro] images not being optimized

Hi My site is really slow. I've installed Hummingbird and it recommends I compress a number of images. Smush pro says it has compressed all of my attachments already (I ran bulk Smush with super-smush enabled) The ones that Hummingbird says need compressing are ones I've uploaded but I asked Smush Pro to also compress images in my wp-content files. I've tried twice and it I get a progress bar that never progresses past 0%