Keep trying to take them out, be very slow but very firm . You also have to stop the nipping , if you don’t tackle that problem now you will never get them to stop nipping . Chances are when you open the door and try take it out it gets a fright, when i let my ferret out i let him come to my hand rather than towering over him . Also if they are male and female you will get babies , split them or dress them people think that dressing a ferret will stop the aggression .