I have no doubt that there are faulty units around. I seen units which either don't receive well or don't transmit well. Could also be a setup issue or antenna issues. Just like with radios. Some installations work better than others. In my case I had PCAS issues until I replaced my PCAS antena. Could also be some sort of a comparability issue with some transponders if we can identify a trend. I have a mode C Becker and have no issues. I should also point out that I did not necessarily follow the installation recomendations. My PF antennas are telatively close to each other in the nose of my glider close to the pedals and not completely vertical yet the reception is pretty good, even from behind. I've seen installations partially below the glare shield which also work relatively well.

Ramy