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

I also have a Valentine 1 and the owners manual recommends mounting radar
detectors high on the windshield and toward the center. They say "Detector
performance is enhanced by a high mounting position. Two reasons. For radar,
a longer sight line to the horizon always helps. For laser, moving away from
the hood and its sun reflections helps a lot."

They go on to say that solar barrier windshields used on some imported
luxury cars prevents the detector from looking through the glass. I don't
know if this applies to you, but some ammunition for thought.