Double check your cable tension adjustment. Shift down into the smallest gear, and turn the tension screw in (relieve tension). Then, shift up, the bike shouldn't shift. Slowly turn out the cable tension adjust screw until it shifts up, then go about another 1/2 turn. If that doesn't fix it, we'll take a look at derailleur hanger straightness. And, if you're not quite sure about the B screw, that could be a likely cause as well.