I noticed an incompatibility between WP Smush Pro and another plugin called WP Media Folder. (Using WP4.4, php5.6, Smush Pro 2.1.2, WP Media Folder 3.3.3)
In case you don't know WP Media Folder helps sort images into folders and when it's active Smush pro's Bulk Smush buttons do not show. As soon as I deactivate WP Media Folder I can see and use the bulk button(s).
I have just let the author(s) of WP Media Folder know of the issue, is anyone able to contact them and maybe get the two plugins working together?