Chick has no leg strength but otherwise seems healthy

[WTChicks]

I've had 15 chicks for two weeks. My one RIR is still very small and has always been a loner. She has very little leg strength and loves lying under the heat lamp. She had pasty butt twice at the beginning. This chick eats, drinks, poop looks normal, peeps and chases others if they have something she wants, but then lies right back down. A week ago she was 34 grams and is now 44 grams so she is growing but very slowly. I started her on nutri drench and boiled egg yolk yesterday and she consumes both. She is on medicated crumble. She has both nutridrench and ACV waters. Her wings and tail are growing in but her wings droop and she doesn't seem to have control over them. I've saw her flap them once yesterday. I noticed yesterday she still has a tiny hard black piece of her umbillicus and I can feel a small hard mass behind it, under her vent. Her vent pulses and is a little swollen so I've kept it clean. Since she's so small, I put her in a smaller brooder two days ago
because the other chicks were running her over and stepping on her. I brought home a tiny cuckoo maran yesterday to keep her company and this hatchling isn't much smaller than her. I thought she was a failure to thrive case but she's seemingly healthy and is growing. I've been doing "physical therapy" several times a day making her stand to get her stronger. No spraddle leg. Her thighs are so thin and I can feel the bones around her tail but she looks like a fat downy chick from the front. Any suggestions or thoughts as to what's wrong? Pictures are from a few days ago and her tail feathers have grown since but she lies like this.

 

[WTChicks]

I've had 15 chicks for two weeks. My one RIR is still very small and has always been a loner. She has very little leg strength and loves lying under the heat lamp. She had pasty butt twice at the beginning. This chick eats, drinks, poop looks normal, peeps and chases others if they have something she wants, but then lies right back down. A week ago she was 34 grams and is now 44 grams so she is growing but very slowly. I started her on nutri drench and boiled egg yolk yesterday and she consumes both. She is on medicated crumble. She has both nutridrench and ACV waters. Her wings and tail are growing in but her wings droop and she doesn't seem to have control over them. I've saw her flap them once yesterday. I noticed yesterday she still has a tiny hard black piece of her umbillicus and I can feel a small hard mass behind it, under her vent. Her vent pulses and is a little swollen so I've kept it clean. Since she's so small, I put her in a smaller brooder two days ago
because the other chicks were running her over and stepping on her. I brought home a tiny cuckoo maran yesterday to keep her company and this hatchling isn't much smaller than her. I thought she was a failure to thrive case but she's seemingly healthy and is growing. I've been doing "physical therapy" several times a day making her stand to get her stronger. No spraddle leg. Her thighs are so thin and I can feel the bones around her tail but she looks like a fat downy chick from the front. Any suggestions or thoughts as to what's wrong? Pictures are from a few days ago and her tail feathers have grown since but she lies like this.

She breathes a little heavy/fast but no wheezing or panting and her beak stays closed.

 

[Ebony Rose]

Nutritional deficiencies start IN THE EGG. If her mother was lacking something, then her chicks are too. With a weakness in her leg(s), I'd be looking at a vitamin B deficiency, further exacerbated with the medication in the feed to ward off cocci overload. If she were my baby, I'd take her (and her siblings) off of medicated feed and supplement their water with an inexpensive, easy to administer vitamin/mineral supplement that is changed fresh daily. If you see improvement in a couple of weeks, I'd consider putting the entire flock (adults and chicks) on vitamin/mineral fortified water. I'd remove the vinegar from the water, it tastes like sin and only serves to reduce fungal growth in the water, which you wouldn't have if the water container is washed daily.
If you don't see improvement, then I don't know.

 

[WTChicks]

Nutritional deficiencies start IN THE EGG. If her mother was lacking something, then her chicks are too. With a weakness in her leg(s), I'd be looking at a vitamin B deficiency, further exacerbated with the medication in the feed to ward off cocci overload. If she were my baby, I'd take her (and her siblings) off of medicated feed and supplement their water with an inexpensive, easy to administer vitamin/mineral supplement that is changed fresh daily. If you see improvement in a couple of weeks, I'd consider putting the entire flock (adults and chicks) on vitamin/mineral fortified water. I'd remove the vinegar from the water, it tastes like sin and only serves to reduce fungal growth in the water, which you wouldn't have if the water container is washed daily.
If you don't see improvement, then I don't know.

Thank you! I have regular unmedicated crumble, so I've just swapped her food and she's already picked at it. I've tried everything else so that's a great suggestion.
I would like to hear others opinions on the ACV water. I haven't ruled out an infection and she has the nutridrench to drink so she doesn't have to drink the ACV if she doesn't like it.
I change water daily.
I appreciate your suggestions!