I made a fairly low-poly humanoid model, and I've succeeded in rigging one side of the model, but when I tried to rig the opposite side, the mesh wouldn't move with the armature's bones (at least not that much). I've attached the blender file, and also attached an image displaying the problem. Does anyone know how I could fix this?
The arm has some bad weight painting. If you look at the vertex weights panel you will find that eight bones effect the mesh movement for the arm, the most effect comes from Bone.005 which is the clavicle - between the neck and shoulder, and even a leg bone on the opposite side of the character is in the list. Bone.007 at the end of the arm only contributes 0.118 to the movement.
Use the panel to remove the effect from the unwanted groups. The copy button at the bottom will copy the weights from the active vertex to the other selected vertices. Note that it only copies the weights, any zero weighted bones will remain in the list and not be removed completely.