Since you discovered the others have an infestation, it would probably be more economical if you get the Permethrin concentrate and treat them all plus your housing.
You can find it at Tractor Supply.
I'm sorry that Tink is not doing any better. Hopefully the vet can point you in the right...
I really don't know why she is so much smaller - she could be younger than you were told, could be genetics or she just is not thriving like she should -there's just really no way to know.
All you can do is see that she is eating a balanced poultry feed at least 95% of the time.
Limit treats...
It's good that she is eating now.
You don't feed chick starter - exactly what are you feeding - Layer Feed?
I would also offer her some scrambled egg. Personally at only 4months old since she doesn't seem to be growing like she should (?) I would feed her chick starter or an all flock type...
Did you get a Permethrin based poultry spray or dust to treat her with? Cleaned out and treated her housing too?
Photos of her poop and where you keep her would be good too.
What are you feeding her?
Are you providing supplemental heat for her and if you are, does she have room to get away...
What a cutie!
Listen sweetie, you are going to have to get rid of those bugs they are dragging her down. Get a Permethrin poultry dust or spray and treat her. Since she is not well, the dust may be better. Work the dust into her feathers, focus under the wings, neck and around the vent.
The...
I agree, get a Permethrin based poultry spray or dust and treat all your birds, clean out bedding/housing and treat that too.
You are concerned about this one pullet - tell us more. Is she lethargic, is the infestation especially heavy, what does her poop look like, how old is she, how long...