Conservation of Mass In Laminar Flow study

Hello,

I have constructed a model for which one of the physics is laminar flow. The model consists of two inlets which eventually join together to create on cylinder. This cylinder has several locations where capillaries branch of and ends with a bifurcation. All of these branches eventually come together at a cylinder located some distance away. When I run my simulations I observe that the sum of the mass flow rates at the two inlets is acceptably close to the mass flow rate at the combined outlet. However, I have some values that I am trying to calculate, and these values are dependent upon the flowrates in each of the branches. Looking at these flow rates I noticed that the sum of the different branches does not equal the sum of the inlet flow rates nor the outlet flow. Could anyone explain to me why this would be. Pretty much why would there be decent conservation of mass at the inlet and outlet, but not within the other parts of the model?

Thanks

Pat