Vex: Phase Familiar targeting issues threshers

I'm in the Timid Kyle's auger mine and I just can't get Big Trouble out at all. Not once in any of the big fights could I get the targeting right to activate the action skill. I don't understand what the deal is there, as I've never had it be this picky on any other type of enemy after 50 levels. I would get similar problems with Amon's rocket fist occasionally, but not this bad.