strangely rigging problem

Hi everybody! I I just rigged my new character. I build a scelleton and then I parented the Armature to my model by “creating from bonehead”!
The rig works fine except for the left arm. I don´t understand why, because the right arm works fine and its the same model and constelation of bones as on the left arm. Nevertheless the left arm is kind of warping when moving its bones!

I made two screenshots:

Please can anybody help me?

You can click the shoulder bone put it in pose , shift click the mesh and select weight paint, The shoulder mesh should be red. If not just paint it . it will go into place as you paint. If I understand correctly.