WP Smush Pro not Smushing

Last year I signed up as a WPMU DEV member, just so we could gain access to your plugin WP Smush Pro, in order to maximize our speed and performance.
We have a fantastic hosting service with WP Engine and they told me today that it seems your plugin is not doing the job for us… and that our images on most pages of our website are not optimized.
Can you tell me why? I'd be most grateful if one of your senior technicians could take a look at this for us.
I signed up for this plugin because it was recommended as the premium plugin guaranteed to produce blazing fast speed.
Please let me know what to do about this?