No streamline is shown when "Magnitude controlled" streamline plotting

Hi,

I've got something I can't figure out during a simple laminar flow simulation with COMSOL 4.3, so I ask anyone's help for this problem.

The problem is that "sometimes" I cannot see any streamline when I used "magnitude controlled" logic for 2D streamline plot. It happens randomly among exact same simulations, but different geometry. I tried to adjust the density of streamline positioning , but didn't work. Once there was no streamline, I couldn't get any streamline despite of any actions I did.

However, if I change the plot logic from "magnitude controlled" to "uniform density" or "start point controlled", I can get the streamline plot. I have no idea what's going on my simulations...

My system is shallow approximated 2-D geometry laminar flow model. The channel consists of some non-circular posts and channel area. The boundary conditions consists periodic flow condition on both sides of the channel, one inlet, one outlet, and no-slip condition on rest of the wall in the geometry. Inlet condition is "average velocity" and outlet condition is "pressure, no viscous stress". Mesh size is "extremely fine" for fluid mechanics.

Does anyone have idea about this problem or any suggestion?

Thanks,
Jaesung