UMAT code

Dear all,

           I would like to ask you something about prof.
Huang's subroutine. I want to verify the code, by using published
stress-strain data. But when I am plotting my stress -strain curve I am
always getting a straight line instead of a curve that shows the hardening effect. What could be a reason for this problem?

         Regards,

         Panos

p.s. I am using a single crystal, single element, and only modify the crystal orientation