Pokemon Black & White Ev training question?

I am ev training my shellder in attack right now. I just beat 30 lillipups/patrats with a power bracer on, so therefore i should have 150 EVs in attack right now right? yet when I go to use my 10 proteins to get to 250 Evs, it says it won't have any effect. What am I doing wrong? (by the way this is my first attempt at ev training...)