It was a demo-car, so I can not confirm that it has been fixed. But the reason I was told was that the number plate was mounted to low, so that the lower part "blocked" the radar view. So you should check this first.
I also found out that they have changed the design a little little bit. The first version (I think), had the radar "behind" the "plastic grid", and the others (I think newer) have it mounted "inside" the grid. What version do you have?
I will get my car in 4-5 weeks, so can not check before then. (ordered it in february 520d...)