UV Warp modifier not playing nice


Can anyone point me how to get the UV warp modifier to work please? I have a small plane set up for the characters mouth and a seperate plane for the selector. I have a warp UV mod on the actual mouth pointing to a base bone and target bone. When I move the target bone it jumps to different mouths as intended, but it selects the wrong mouth.

Ive tried setting scale and rotaton of planes and bones. What am i doing wrong?