hello ,
my unit is a " valentine one " and occasionally
i experience the same situation .
perhaps you have NOT encountered any
" real " radar signals .
your unit may filter out any " false radar signals "
such as stores , banks , etc. .
if you drive into a mall or a bank parking lot
you SHOULD get some kind of signal .
i've NEVER heard of a windshield obstructing
radar signals .
in my opinion , Valentine One is the BEST
unit available and has saved me MANY ,
MANY times . it's the most expensive because
it REALLY works !
radar detection is NOT something i want to
compromise on .
                         regards ,
                           Dick

My radar detector hasn't beeped since I got this new 2008 328i.  Every
once in a rare while, the lights will flash but hardly ever.
  I have it mounted inside the windshield in the center below the rear
view mirror about 3 inches above the dash.
  My car was in the shop last week and they gave me a new 330i to drive
while I was waiting and it did not work in that car either.
  I suspect it is some type of coating on the windshield, and if so,
how can I get my detector to work again?  If my radar detector doesn't
work, I might just as well be driving a Yugo(;-]

TIA