Countor data set does not generate plots

Hi, I´m solving a very easy Laplace equation on Comsol, I would like to extract values of some variables I´d define along the isolevels of another user define variable. I´m able to create the Contour Data Set for u (is the Laplace ecuation solution) but when I try to create this data set for ux (derivative of u) or using any of the variables I´d declared in the definition section the result plot of the isolevels is blank. The results of the problem is ok, I´m also able to Plot the iso levels of the defined variables on a Contour Plot. Any ideas?

Regards

Agustin