This should help you. It gives you a nice frame by frame walkthrough to clearly illustrate what you need to be doing.

Animation -> Movements -> Jumping

Two and a half problems, as far as I can see.

One issue is your starting pose. It's much easier to start a jump with the legs a bit closer together, and crucially you need the stick's body to be more central. Right now he seems to have most of his weight in front of his front foot. If you try standing like he is you'll find it quite difficult, and any jump will be more of a hop. Make sure his knee is behind his heel.

One and a half issues are the stiffness. You're not bending down far enough in the anticipation, you're not swinging the arms enough, and you're not moving the legs enough when he's in the air. And I don't think you're straightening the legs enough as he pushes off. Basically, you're not generating enough power to push his body as high as it's going, so it looks wrong.

http://s1248.photobucket.com/albums/hh491/Cythe0/?action=view¤t=Jumper.gif
Look at my third try.

That's much better. If I were you I would move the arms the other way each time, so the arms should go down/back before the jump and come up as he's jumping. You also need to try to bend his back a bit. You might need a new stick if there are no back joints in that one. Lastly, take a look at this. It'll help with foot placement.