Stress-Relaxation Graph from a Simple Hyperelastic Model

Hi,

I am trying to get stress relaxation profile from a simple hyperelastic model.

Step: I applied 2 steps(static, geometry) in a simple 3D cylindrical model where first step was 30 secs and second was 180(fixed/incremental) secs (exactly same like the experimental data).
I also ran the simulation in 1 step(210 sec) too while I used experimental data as amplitude in BCs

Boundary condition: Then in BC, I applied 20% strain to the z axis(on the top of the cylinder) with ramp for the first step.
in the second step I used propagated/same distance with ramp/ same distance distance with constant ramp.

Field Output: I used reference point to get force vs time graph and element to get stress vs time graph.

Graph and Issue: The issue with the graph is - Though I am getting the loading portion perfect, the profile in the relaxation stage is showing straight (no change of force with time is constant), it is not going down with time.
What is the wrong with my procedure? What may I do to get graph like the stress relaxation graph from a hyperelastic model?

Thanks in advanced.