Armatures not staying in model?

Hello, Blender community! I’ve had Blender for a little less than a month now and I seem to have run into a problem with my armatures. The bones don’t follow always follow the model, is there a step I’ve forgotten or is in just not rigged correctly? I’ve tried using weight painting, and it helped a little, but not entirely. Here’s what I mean:

If it helps, I’m running on Linux mint and my Blender version is 2.63.0

There are many causes for this kind of behavior. First thing I check is the Armature modifier on the mesh object. There should be only one Armature modifier there.

I do have only one Armature. Could you tell me some of the other possible causes?