1.) You're body has become accustomed to your workout.
Your body is smart, it grows and becomes stronger as we challenge it. But there begins to be a point after you've run the same route, lifted the same amount of weight, done the same workout body where your body begins to "yawn" at your workout. Your body has done this same thing a million times before, there is no challenge anymore. How many times would you want to take the same class, watch the same movie, listen to the same song until you want to roll your eyes and yawn. You already know it, and it has become a part of you but offers no new challenges or excitement. Your body needs to be challenged.
Try something new! Break out of your routine! Make your workouts shorter and more intense, do HIIT workouts, add resistance to your cardio, go heavier in your resistance training! You want your body to be challenged and shock it out of it's routine to see change! When I was just doing the same workout of running every day on my elliptical, which is still burning calories, but not showing change anymore I wanted to change it up. It got a little mundane and I wasn't seeing a whole lot of change. I started Insanity, and boy...was my body shocked. That is a workout that does NOT mess around! I also started shortening my time on the elliptical and doing HIITs on it for 20-30 min.