What was the viscosity ratio between the phase that was displaced (low viscosity) and the displacing phase (high viscosity)? Viscous fingering only occurs when there is a large enough ratio.

 I agree with Alan's comment. You can try to use larger viscosity ratio, and use the low viscosity fluid to displace the high viscosity one. And you can try to use a porous domain.

Oops, thanks Xiang Lei. Of course - the higher viscosity phase displaces the lower viscosity phase if you want to see fingering. My mistake.