80 pounds is big for a female. How tall is she? How active? Can you see any of her ribs? Can you feel them easily? Maybe she’s just not hungry enough to eat.
If she doesn’t eat breakfast after 20 min, just pick it up and offer her the next meal at dinner time. If she doesn’t eat dinner, do the same - it won’t kill her to go a day without food, especially if it’s her choice. If she eats dinner and turns down breakfast again, you might just be feeding her too much, and she’s self regulating. If she misses a full day of food and still won’t eat, could still be that, but more likely something else is going on.
If she gets too skinny but still won’t eat, talk to your vet.