Moving surface charge - pzd model

Hello

I tried to build a simple test model of an oscillating surface with an applied surface charge (with pzd model).
The solution shows that the surface oscillates correctly (displacement plot). But when I plot the potential no movement is shown. Furthermore, when i evaluate the potential at a fixed point in space near the charged and oscillating surface, the potential remains constant, as if the surface doesn't move at all. I would expect to see the potential oscillate with the same frequency as the surface.

Does anyone know what i am doing wrong?

Model is attached...

Thanks in advance