Specific *possible* causes of embryo issues starts around page 51, and chick stuff on page 52 ish..The chick also does not have feathers on its back or neck. Would this be something breed specific or an issue with the chick?
https://www.hubbardbreeders.com/media/incubation_guideen__053407700_1525_26062017.pdf
I don't think it's breed specific.. more likely breeder diet, possibly a single defficient hen IF everyone else is fine.. and it wasn't incubation temperature related (which day 21 sounds pretty good).
Nutritioinal deficit in the hatching egg.. supplement as suggested by other posters already..Chick does not stand up on legs and feet are curled inward. Could this be slipped tendon or something else?
The "shoe" fixes symptoms which *is* helpful long term (regarding roosting, etc).. but max benefit will be seen when treating cause also.. for anyone who reads this in the future, since I see you are already addressing it.
Please note that electrolyte supplements may not contain enough of the ribolfavin (E or what have you) to make it count for a bird in perril. While B vitamins don't overdose easily.. some other stuff might.. my point being that sometimes a targeted vitamin might still be the best choice.. when looking for more than just a little pick me up.
Hope the little darling shows improvement and recovers quickly!
