Narbuk stage 7 consistently only gives me two stars when i beat it with two heroes without losses.

it has happened in the past on every difficulty. currently i am on the hardest difficulty with vogoth and centurion at level 60 ascension 6 with immortal and regeneration gear. i have tried three times and each time has ended the same way. achieving a win with one hero gets around this, but on nightmare difficulty, that is a tall order.