Falloff only affects half the mesh?

I have something happening that I don’t understand at all and hope someone can help with - I’m working in 2.59 and the falloff (both connected and enabled) only affects half the model. No matter what the radius influence, half of the mesh behaves beautifully, the other half doesn’t move or change at all. Selecting faces, vertices or edges makes no difference. I modeled the mesh as a half, added and applied the mirror modifier, and the half of the mesh that is NOT responding is the original model (left side), not the mirror (right side - which IS being affected by falloff correctly). Please see photo where I over-compensated the influence to better show how one half of the mesh is adjusting while the other remains in place.

I’m creating shape keys and the lack of influence is the same if the mesh is in basis, or keyed positions.

Going into falloff mode disables the X-symmetry option, so you best bet is to make sure you select both vertices (or edges, or faces) on both sides of the model.

Thank you, I’ve already applied the mirror modifier and am working with a symmetrical model so the exact same points have been selected on each ‘half’.