Yesterday, I was having this problem. Taking the advice, I messed with the barrel adjusters. Now, every gear is working great except the highest gear and lowest gear. I put my bike in the highest gear and I can still click the shifter once more, but nothing happens. When it comes time to shift down gears, I have to click the shifter once before shifting has any effect. In the lowest gear, the chain jumps. I have to hold down the shifter to stay in gear there.
Here's a video that explains how to adjust your rear derailleur. Since you didn't mention adjusting the high gear or low gear adjusting screws, this may be your problem area. Sometimes it's best to start with all adjustments in a "neutral" condition before you attempt to "fine tune" the derailleur. It would benefit you to tell us the brand and model of your deraileur syatem the next time you have a problem. Even different models from the same company, can have their little "quirks" ofetn learned by experience rather than instructions. Hope this will help you out! Good luck!
Here are a couple of links that explain derailleur adjustment: