Variation in Factor of Safety for a slope with different time duration of rainfall???

Hello, 

I am trying to investigate the factor of safety for a slope with different time intervals (1h, 3h, 6h, 12h and 24h) and rainfall (q=3mm/h) with soil permeability (kx=ky-0.6972E-3). After running analysis, there is no change in FoS and displacement values, they remains constant. What could be the reasons for this? Anyone who can help me, please response.

Thanks