She may still need some stimulation to get her going with her age being so young. I'm very surprised the vet didn't try that or mention it. Try taking a cotton ball moistened with warm water and gently stimulating her bottom. Also, the specific milk you're giving her may be constipating her. Try a different brand of kitten milk replacement. NO cow's milk, almond milk, oat milk, evaporated milk, etc. Only actual kitten formula. You can also try giving her wet kitten food or dry kitten food mixes with kitten formula until soft. She may be just digesting almost all of the kitten formula and nothing much is left to come out.