Basic issue with plotting derivative

Hi everybody,

I am using the 2d structural mechanics for static simulations of a cantilever undergoing a point force. I want to plot the derivative of the deflection u by respect to the position x along a boundary. But plotting d(u,x) gives me a zeroed value function.
By digging in the forum, I thought first that this may be due to the order of the shape function, but it should not be an issue in my case as I just want to know a first order derivative. It looks like a basic issue but I do not find the solution.

Thanks