why my umat's behavior degrade in parallel mode?

I've met some strange problems implementing umat. To speak in short, it's parallel degradation, which means when run in serial mode, everything is OK, there's no failure of equilibirium iteration and therefore no cut back, but when switch to parallel mode, there are random cut backs. I checked the process by outputing results to files and found very bizarre phenomenon, the data changes when transfering from the subroutine called to its caller. For example, if the main program calles sub(a), and in sub a is specified intent(out). The subroutine returns and assigns a=1, but after returning to the main program, a=2 not 1. The value changes during the combination of dummy and actual argument. This is totally nonsense! After I uses pointer to transfer data, things get much better immediately, but there's still random failures of equilibrium iteration and I don't know why. Can anyone help me out? Thank you!