Airspeed Indicator Strangeness

Yesterday on the takeoff roll, I discovered my airspeed indicator was
stuck. I got off tow a little early, and returned to the field to
sort things out.

Back on the ground, I visually inspected the pitot tube (in the
vertical fin on my Ventus 2bx), removed and blew out the short pitot
tube, and put it back on. Then I verified that the airspeed indicator
responded correctly when I (carefully!) blew into the pitot tube.
Problem solved (I thought)

On tow again, I found I had the same problem - hmm. OK, I'm an
experienced soaring pilot, and I don't need no stinking airspeed
indicator, so off I go cross-country. As luck (or lack of airspeed
indications) would have it, I landed out after 200km, about 30 miles
from home - ah well.

While waiting for my crew, I had plenty of time to troubleshoot the
ASI problem. I repeated the steps I had done before, and verified
that the ASI responded correctly on the ground - how could this be?
Then I had the bright idea that the difference might be the position
of the instrument panel - tilted up or down in the Ventus. So, I put
the panel down in the flying position, and voila - the ASI sticks!
Tilted it back up, and the ASI works fine. Tilt back down, ASI
sticks. Of course, the original ground check had been done with the
panel tilted up - lucky me!

After a number of other experiments, I was able to confirm to my
satisfaction that no tubing was being pinched when the panel was put
in the flying position, and the only variable was the orientation
(tilted or horizontal) of the instrument itself.

This is a fairly new instrument (circa 2002), so I'm puzzled by this
apparent failure. Anyone have any ideas?

TIA,

Frank