What level mobs are you fighting? Regen will only be cast if the mobs level is about 5 levels less than yours or greater. For example if you're level 30 your automaton won't cast regen if you're fighting a mob level 24 or lower but will if you're fighting a mob level 25 or greater. As a general rule of thumb if the mob is in the upper range for decent challenge (not check low eva or def) you'll get regen.