There are a number of reasons this could be. First, check your battery's status. If you hold down Option and click the Battery symbol you can see the condition of your battery. If it reads anything other than Normal, tell us.
Next, click the battery symbol again and see if any apps you are using are using 'significant energy'. This could explain any reduced battery life you are experiencing.
Are you using Google Chrome? If so, I would avoid using it on battery. Chrome is a battery killer.