Help with the Laplacian definition (second derivative)

Hi
I want to do the laplacian of the hydrostatic pressure, which I've defined as the trace of the stress tensor / 3 (solid.I1s/3), but i've a problem,when i define the following expression : d(d(solid.I1s/3,x),x) +d(d(solid.I1s/3,y),y) the result is zero,which is not the expected result. Could you please tell me why is this happening?

Thanks in advance
Edward