feathers growing in really slowly?

this is my hen butters, i got her at a discount because she was missing some feathers. its been about a month since i bought her and her feathers on her back are not coming in. her little butt nub feathers are coming in but for some reason her back is still very naked. is there anything i can do to help them come in faster? ive been waiting to integrate bc i dont want her to get pecked by my other hens