I have a rat grooming question.?

I have two hairless rats. Zeno, the dominant one, will hold Ember down and groom him and that is fine and I know why he does that.But Ember refuses to groom Zeno. Zeno will put himself underneath Ember and wait for Ember to groom him and Ember will just walk the other way and then Zeno get's mad and bites him. I don't understand why he does this. Ember seriously just will not groom Zeno. Does anyone have any idea why that would be?