2.) Do the Math
You need to make sure you are keeping your nutrition in check. It is SO easy to not count the extra handfuls of food here and there. You could be over eating and negating your caloric burn and hard work. Now, I don't want that to come off as if you are eating more than you are burning there's no point in trying, or give someone an excuse to give up. The process will just be slowed or become stagnent if you are over eating what you are burning. The more fit you become, the harder you have to work at staying fit, it's just how it is. Someone who carries more weight is carrying that extra force in their workouts, making it much harder and makes it so they burn more calories in exercise versus a smaller individual due to the fact they aren't carrying all of that weight and their body isn't being challenged as much. You have to add intensity to your workouts and push yourself to burn more and keep in check with your input and output. You can do that by getting some kind of calorie counter. My recommendations are in my fit tips section of the blog.
This is sort of a more rare situation but, you also may not be eating enough to keep up with your workouts. You could be over burning what your intake. Everyone thinks that if you starve yourself you will lose weight and be skinny. Well, you may lose a few lbs, your body will feel as if it is starving and lock in defense mode. It is built to fight the elements and survive as long as possible. Once you are in starvation mode, your body will hold on to everything you do eat and store it as fat. Then your metabolism slows as well, and your metabolic rate burns less than it did before when you were eating. Fueling your body withthe right foods keeps your metabolic rate pumping. You may also be over exercising your intake, leaving your body exhausted, tired, weak, etc. It isn't getting the nutrition it needs when you are working out, it can't get that energy and keep the body strong and durable for your workout or to recover for that matter because it needs nutrients to recover the new muscles you build or strengthen.