This sounds about exactly what my hen was doing. Since there were several of us that were having a similar problem at the same time, it was sort of heartwarming, in a strange way, to think of all of us BYC'rs all running out to various stores to get the Vitamin B.
I got mine in capsule form, and poured the powder in or on everything I put in front of my chicken. She was in a pen by herself, yet out where the others could come over to chat with her. She drank and ate pretty well. I gave her oatmeal and boiled eggs, along with bits of fruit of various kinds, and always had her chicken feed in front of her. She would scoot herself around to get to her food, and was always alert.
It took about a week for her to start trying to stand, and after she did that, then it was another 2-3 days before I put her out to join the others. By this time she was almost back to normal...still a bit shaky and not ready to jump up on the roost, but was improving daily.
I was honestly floored that this worked! I was just so afraid it wouldn't, but was motivated to keep at it since others were having good results.
I've told several people about this....people who have an interest in chickens (feed store folks, vets, etc.) and no one had ever had this experience with a chicken. I wanted them to know, just in case someone should approach them with this same problem. At least they would know of something that might/probably would work.
Good luck! Please keep posting so we will know of the good (hopefully) progress with your little buddy.