I suppose we're hoping that bombarding the body with unnecessary nutrients will cause it to store the excess in hair and nails, in effect, making them grow faster. The problem is that most American bodies could darn well use those nutrients, and the body does not "waste" these resources on hair and nails. If the person is taking in a healthy, nutritious, adequate diet PLUS vitamins, at this point hair growth should occur.
Again, it can take 1-2 months for vitamins and excercise to become visible on the outside; give your body time to finish what it is doing before deciding it's a bust.