Handheld GPS Works Fine, Except in One Particular Aircraft

When I attach it to the yoke and put the antenna on the dash over the panel,
it refuses to get any satellites--- none. Yet it works fine in another 172
and in a Taylorcraft -- and in all the other airplanes I've used it in.
Now why is that? It will work as long as the master and radios are not on
and the engine not running. It's a Trimble Flightmate Pro, ancient but
adequate for me. TIA for satisfaction of my burning curiosity.