Cannot get Creep Strain with reasonable Creep Strain Rate? time integration stopped ?

Hello all, I have been struggled with a weird problem for quite a while:

I am using a user-defined Creep rate tensor in the the Solid Mechanics Module, in order to simulate material response that depends on strain rate.

I works well for most of the time, but recently, sometimes when I apply a boundary load on the model in my time-dependent problem, the model gets very reasonable creep strain rate (inelastic) at each iteration, but the creep strain is always zero at every time step.

It seems COMSOL has stopped doing the time integration of the creep strain rate in order to get the creep strain.

I am wondering at the cause of this problem, and how can I solve it ? Is it due to any special setting in the solver ?

Thank you so much for your help.

Best regards,
G.Lu