What Susan says is true- sometimes it needs a few extra round "between" gears in front before it picks itself up to the next gear. When this happens to me it's often because I've pushed the lever a bit TOO far and am overshooting the goal. I have bar end shifters, but the problem sounds similar. Try to let up on your pedalling force right as you shift- just sort of float your feet during the actual shift process- pedalling hard makes the gear/chain change more problematic.