Re: Why isn't she eating?
This might seem like a silly question, but are you feeding her with a bottle or a sippy cup? I'd try the sippy cup. Or a regular cup and see how she drinks.
My daughter barely drank anything when she was an infant. Like you, she was topping out at around 15oz when she was 5mo, and drank wayyyy more when she was a newborn. By the time she hit 7mo, I gave up the bottle altogether during the day. It just wasn't happening.
Also, are you feeding her solids? She might just be ready for them. If you are feeding her solids, I'd give her as much avocado as she'll let you. It's packed with fat & calories.
If she keeps it up like this, then you'll be in the same boat that I am. My daughter is 14mo now, lost a couple oz at her last check-up, and won't drink milk period. So, instead of sleeping like a normal person, I wait till the middle of the night to dream feed her. (It's the only time she lets me dream feed!) And to keep her calories up- the bottle I give her is 1/2 heavy cream & 1/2 whole milk.
Good luck. I really hope her appetite picks up.