Did you use MonkeyJam? If you did, did you take the frames and put it at 15fps? Don't do that, you see, I used to do that and noticed that it doubles the framing, which means you are actually going at 7 and a half fps. There are two things you can do, the hard way, or the easier way.
Hard Way: Shorten all the doubled frames to 1 frame and then put it at 15fps. I don't do this though because if you have a huge piece of animation it means you'll have to shorten every frame and move it, which is very stressing and time consuming.
Easier Way: Keeps all the frames at doubles, then put the fps at 30fps, and it will be 30fps at doubles, which is just 15fps.
I do it the easier way. Easier for me.
"Animation is about creating the illusion of life. And you can't create it if you don't have one." - Brad Bird