VUMAT shear loading

Hi everyone

I have written a VUMAT code for Isotropic -Viscoplasticity material using the finite strain theory and matirx "F"

the code works properly for compression and tension test, however , under shear loading it work just properly if I use only one single element to model the specimen.

when I increase the numebr of elements under simple shear loading the model starts to oscillate until similation aborted.