how can I deal with the thickness strain in the Shell elements (S4R) writing a VUMAT subroutine ?

Hey buddies

I am writing a constitutive relation in vumat for plane stress conditions with S4R elemet in the ABAQUS.

the problem is that the E33 is reported as zero where it is not zero and also the values for S11 and S22 are so large numbers. 

ihave tried the code for 3D conditions and it work great with C3D8R element and the results are satisfactory where for the Shell elements by enforcing the S33=0, the E33 gets zero too.

does anybody have any solution? 

thanks alot