That's because she is sick. You do know for a fact that your A&M is a hen?
99.256% probability she is sick. 1.069% probability that she is broody. There is a 62.53% probability that neither you nor I knows what sex your bird is.
To answer your basic question "Why does my Texas A & M quail always stay in her nest box?"
More info is needed here before we can jump to a conclusion of illness, although that is a possibility.
How large is the pen?
How many birds are in there?
How many males?
How many females?
What style of construction is the cage? the nest box?
How old are the birds?
How did you determine that she is a female? (Males can and will mount other males, so seeing one be the "bottom" is not proof of gender.)
I am wondering if it might be hiding to try to avoid another more dominant bird (or birds) that it is afraid of...
1) What type of bird , age and weight.
2) What is the behavior, exactly.
3) How long has the bird been exhibiting symptoms?
4) Are other birds exhibiting the same symptoms?
5) Is there any bleeding, injury, broken bones or other sign of trauma.
6) What happened, if anything that you know of, that may have caused the situation.
7) What has the bird been eating and drinking, if at all.
8) How does the poop look? Normal? Bloody? Runny? etc.
9) What has been the treatment you have administered so far?
10 ) What is your intent as far as treatment? For example, do you want to treat completely yourself, or do you need help in stabilizing the bird til you can get to a vet?
11) If you have a picture of the wound or condition, please post it. It may help.
12) Describe the housing/bedding in use
Maybe She Is A He And All You Need To Do Is Unplug The Tv... Then He'll Get Bored And Come Out
Seriously Tho Sex And Set Up Info Would Help Make A Better Idea. There May Agression Problems And Thats The Bird's Safe Haven? Or It Could Just Be The Bird Actually Likes It There? Or 1 Of Many Other Things?