You're going along great, you lost 20 pounds, and next thing you know... nothing. It can be one or a combination of things causing this. See if any of these look familar.
1. Loss of lean muscle mass-If your exercise program consists primarily of long, extended cardio exercise this could be why you quit losing weight. The more muscle mass you have the more calories you are burning while at rest.
2. Can't keep up the pace-I call this The Biggest Loser effect. If you've never seen that show, the contestants are put through a crazy amount of workouts, fed all the right foods, and they do great. Have you heard about what happens to a many of the participants after the show? Many of them gain back the weight they lost on the show because it is hard to keep up.
I see people all the time that put themselves on a crazy exercise and nutrition program. Working out twice a day every day, eating absolutely perfect, etc... They are losing weight, doing great, and then it just wears them down.
3. Frustrated by plateaus- Have you got to that weight or size that you just can't seem to improve? Everybody does and this is where many people get frustrated and give up. It is hard, but you can break through. This is what separates the people who have short term success from the people who keep making progress. Do not give up just because you reached a plateau.
4. Body adapts-Have you been doing the exact same thing for years? Something has to change. The exercises you do, your pace, the amount of weight you are using, your intensity, etc... You have to be constantly challenging your body.
5. Restricted calories-People who severely restrict calories will lose weight for awhile, but then they gain it all back plus a few more. For one, they lose their lean muscle mass as discussed earlier. When your thinks you are starving it, it will begin to store any calories you take in as fat. It doesn't know when you will eat again, so your body wants to keep something for the rough times.
Another reason restricted caloric intake is bad, is because you will not be getting the necessary vitamins and nutrients needed to support your body and any exercise program you might be on.