Flow simulation of an assembly, problem with lofted part.

I've altered a design that was made by my previous co-workers, but it looks like there are strange things happening with the air flow simulation. I've added a rear wing to a car design, but it looks like the airflow just passes through my wing. When I simulate an airflow on the wing itself (not in an assembly), the airflow is displayed correctly (with vortices on the wing tips etc.). The wing is a lofted part part which is knitted to a solid, so I don't think it's a surface problem (I've encountered the same problem caused by the car being partly designed as a surface. This is fixed with the 'knit surface' tool). What is causing this problem and how can I solve it? Btw, I'm new to Solidworks.