Smushing Process doesn't start

Dear Devs,

I've successfully installed the latest version of WP SMUSH PRO plugin but nothing happens when I try to smush images. I am running WPLMS 2.9 Theme on WP 4.8. Pressing the blue "bulk smush now" button doesn't do anything. I have tried several options in the settings without success. I am using W3 Total Cache but it doesn't cache pages for registered/logged in users.