I agree with
@azygous having some testing done (fecal float for worms and Coccidiosis) and a necropsy would be very informative.
Putting your location in your profile is helpful when asking for help, a lot depends on where you are in the world as to what products/course of treatment may be recommended.
Photos are always welcome - chicken, poop, housing - whatever you want to share.
What type of food/treats do you feed?
How old is she?
You've treated for an impacted crop - was the crop impacted? Is she pooping - what does that look like - is her crop emptying overnight?
Does she present with any other symptoms besides lethargy - laying soft shell eggs, difficulty walking/paralysis, fluffed/cold appearance, molting, etc.?
Since you have had so many die in short order, then check your feed, scratch or anything else for molds, fungus, anything rotten or toxic.
She is losing weight, so regardless of how old she is, it may be best to feed chick starter or a flock raiser formula that has at least 18-20% protein to see if that helps. Limit treats like scratch grains or eliminate them altogether until you can figure this out. Some fresh greens, egg, tuna or veggies can be given as treats.
It's very possible you may have something like Coccidiosis that is taking it's tole. Treatment is with Corid which can be found at most feed stores, but getting a fecal float will give you a better clue. Unfortunately, diseases like Marek's can also cause wasting and there is not much you can do for that.
I'm sorry that your first post to BYC has to be under these circumstances, we are glad you are here and that you joined. We may not be able to give you all the answers, but there's a lot of people here that are wishing you and your flock well and hoping they get better.