Ask for help, numerical resolution of Richards equation using Van Genuchten model

Hi everyone ! I am working on predicting pressure head using Richards equation. I recently read about SimPEG and I tried to run some examples from https://docs.simpeg.xyz/content/examples/20-published/plot_richards_celia1990.html#sphx-glr-content-examples-20-published-plot-richards-celia1990-py
my problem is when I tried to change the parameters from Haverkamp model to Van Genuchten model (initial and boundary conditions from celia1990), the output seems to be constant and equal to the initial condition. Thank you in advance.