if it has nothing to do with stuck eggs, another clutch, and the fuzzy idea doesn't work, I usually give a few liquid meals to build her up. I have had some females (and even an occasional male) after the breeding season that go into a downward spiral of either regurging or not eating, and they can die from it. My unproven theory is that once they get rundown, the "bugs" that are always around (Salmonella, who knows what else?) multiply and take over, becoming pathogenic. By giving tiny, frequent meals (fuzzies or liquid), they build themselves back up and the "good" microbes again start to outnumber the "bad guys". I have brought a number of them back from the brink by doing that over the years, and I have lost some that I didn't start working with soon enough.
If they won't eat fuzzies, I usually use a ground up canned cat food, mixed with water and fed through a tube. I start with just a few ccs of watery gruel to get them started, then mix it heavier as the feedings progress. Since I have started adding Nutri Bac to my arsenal, I have better success than earlier, which to me reinforces my whole theory. After a few weeks go by of twice weekly fuzzy or liquid feedings (plus Nutri Bac), I start trying them on their own, and increase food mass.
I am not a vet or microbiologist, but this is what has worked for me, and the experience that has led me to form my theories.