Okay that is good to switch so you know she is getting the full dose of the B complex. We can't know for sure if this is a niacin deficiency but if she doesn't show improvment after you know she is getting the 1 ml then after a week or more we will know it is something elses. But with her legs going out [lameness] it sure sounds like Niacin and Pekins are so prone to it since they grow so fast. Don't give up on her just keep up with the b complex also water therapy even if you have to take a towel afterward and pat her dry water therapy will help her build up her muscles. [If it is cold where you are I'd do water therapy in the tub with warmish water to keep the muscles loose. Once she is feeling better she'll begin to waterproof herself. Also she needs her starter more than veggies becasue the starter has all the nuutrients she needs so keep veggies right now just for her b complex. There is another prodcut called Rooster Booster "Poultry Cell" which will be good to add to her water to all the duckling's water actually. It will help to give their immune system a boost. And should really help your little one that is having problems. Just make sure it's Poultry Cell.
Hopefully this a is just a niacin deficiency and she'll recover. The liquid B complex has sure helped many ducklings get over it we just have to be patient and give them support while it is working.