Photos of her and her poop?
Is she molting?
Any lice/mites?
If she's not molting and hasn't laid an egg in 3wks and should have, then she may be having some reproductive issues. Hard to know. Cancer, Salpingitis, EYP, etc. can be common in laying hens.
Sometimes the best you can do is work on getting them hydrated, then attempt to get them to eat their normal feed.
It sounds like she's willing to eat veggies and scratch, so check the feed just to make sure it's not off, moldy, rancid, etc.
Soaked feed made into a mush may be more enticing. Sprinkle bits of egg, fish, etc. on the mush to see if it interests her more. If in molt, she may just be off her feed which some hens do, they prefer other things besides their normal feed, especially greens and yes, scratch - they want comfort food I suppose. Give her small amounts of finely chopped up Kale/Collards/Greens and bit of scratch to go along with her normal feed. Provide some grit free choice.
I assume you gave her something like this and her poop "improved"? Did you ever see any worms in the poop or signs of canker inside the beak? Just curious.
A lot of people do use this to see if it helps at all, sort of cover all type of thing.