Difficulty with weight paint mirroring

I’m having trouble getting the X-mirror to work with my model – at least to work consistently. The problem is with the spine of a human character. I would like to have Blender mirror the right and left sides of the torso and waist. However, it doesn’t seem to be working. The neck and pelvis areas will mirror just fine, but the vertices tied to the spine bones supporting the chest and abdomen refuse to do any mirroring. I’m at a loss to find a way to fix this. Is there an option I’m not seeing? (The X mirror and topo is turned on, and the model is fully symmetrical about X.)

Blender 2.75B

Never mind. The problem was that the mesh had a very tiny asymmetry to it. It wasn’t really visible, but the symmetrize tool fixed it.