The biggest problem you've got going on here is the formlesness of the robots. They look lik amorphous blobs. You should really be studying form and light. Most of the shadows and highlights really don't make any sense. And this make them look very flat, and rather strange. Even though you have drawn the forms themselves right, the shadows convince you they're a shape they're not supossed to be. If that makes any sense. Try to observe forms in real life, look up photo references, see how light falls on and influences a form.
I suggest you give this tutorial a read, it's very good and I'm sure it'll be a great help:
http://www.itchstudios.com/psg/art_tut.htmThe forms are not the only problem though. The overall posture of the robots is also greatly lacking. Especially the pose of the blue robot. He has this very strange stance that doesn't exactly make him look like he's ready for a fight.
You should also really take a look at the perspective issues. I know you probably want the blue robot to be symetrical, but currently he looks completely different on the other side because there are a lot of mistakes made in the lineart.
For example, his left shoulder (our view) looks like it's facing backwards, while the right shoulder looks like it's facing forwards.
You should also give the designs of the robots alot more thought. The right one is okay, though not without it's problems. But the blue robot doesn't really make any sense in terms of design. I realise you might want them to look cartoony or whatever, but the joints for example still don't make any sense. There's no indication of knee joints on his legs, which would make it impossible for him to walk. In fact, there's no real indication of any leg joints at all.
Take a look at a few robot designs. Like gundams or Mechwarriors to give you some ideas on this.
Good luck!
