The first thought that comes to my mind is whether you're eating enough. If you don't, your body automatically slows down its metabolism to conserve resources.
What type of exercising are you doing? Sometimes you need to mix it up to restart the furnace, so to speak. Do you check your measurements, or just go by scale numbers? You may be losing inches but building muscle so the weight would not change as much.
I think the important thing is to tweak it up a little but not to give up. I had three weeks at a standstill recently and this week the scale finally moved again. The awesome people on this site carried me through my moaning and groaning and told me to hang in there and it would change, and it did. Just have to listen to your body and not rush it. Good luck!
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days