Vitamins raise EVs by 10, according to the stat it says in the description of the Vitamin. If you don't know what those are, you can look at the EV portion of the site under "Game Mechanics".
So there are 2 possibilities. There is a cap of 100 EVs worth of Vitamins in one stat, and you can't use any Vitamins if you are past or at 100 EVs. So that could be a possiblities.
The other possibility is that you're EVs are simply maxed out, at 510 meaning you can't gain any more EVs. You can check your EVs by calling Bianca and she will tell you if they are maxed out for a certain Pokemon.
Hope I helped! :) Source: Experience