It sounds like it is just out of adjustment. If it was a new bike you bought and you haven't had it adjusted then that is most likely the issue.
Cables stretch and parts wear in and need to be adjusted to compensate for this. I usually get a full tune up on mine at the beginning of Spring and it is good until the following spring as I don't ride inthe winter. If you ride a lot more then you may need to make adjustments a little more often.
There are some good online sources that can walk you through how to do this or you can take it to your LBS and have it done. I usually take our to the LBS as they do a great job, work through everything that needs to be done and do it for a great price. After a good thorough tune up it is almost like riding a new bike again.