Why my budgie couple fight?

i have a male budgie and a female budgie. i know that for certain and they are a total pair and does EVERYTHING TOGETHER. for some reason she does not let him enter the nest box and he finally did and hit the snot out of him and now he is injured so every time she leaves the nest box to go eat she stops him from consumption a picks a fight with him and has no choice but to watch over himself and now stays far away from the nest box. Can someone explain please?
Quaker and cockatiel sound out...??
Answers:    ** If he is smaller than her, she will pick on him. If there relatively alike size, it is probably that he is very timid, and when they fought for the first time, she then realized she could achieve away with being a bully. It may never be any better. Some animals basically will not ever become "buddies" regardless of anything else. **
I want to catch a pet bird.?   On a ascend 1-10 how loud is this bird? 10 points?   What type of bird should i bring back?   Will consumption sand sheets damage my budgie?