Do a basic cable tension check by shifting all the way to the small sprocket in back, and then see how taut the cable is. It should have some tension, but not be tight. It most certainly shouldn't be slack. If there is perceptible slack, loosen the pinch bolt and pull the excess cable through. Tighten the pinch bolt and then see how it shifts onto the bigger sprockets.

You'll probably need to check the high and low limit screws, too, to make sure the derailer doesn't go too far either way.