While they know where the interference is, the appearance changes with the wind and weather. The reflections change as the windmills rotate or reorient to the wind.
It becomes very difficult when there are sporadic storms moving through an area.
zoom in on the radar below and look below the “r” in Beaver Dam. There is wind farm interference, but watch how it changes, appears and disappears.
http://radar.weather.gov/radar.php?rid=MKX&product=N0R&overlay=11101111&loop=yes