>>723516063
Dominator needs you to be between 6th and 9th place and to be Late race, not mid, meaning all the Debuff Natures will start dying mid race bringing your Dominators to the front.
Assuming everyone brings 1 Nature, that's 3 back spots already full, leaving you with just 1 spot left to activate Dominator, and if there are other Lates or End closers that are getting pushed to the front, your Dominator won't activate at all.
If you want a 100% proc, you need to build your Dominator to just suck ass really bad and die before the Natures do, that way she can be last place and activate it late race.