There are plenty of qualified judges out there that are never asked for one reason or another. It's funny how the same people seem to be judging multiple tests in a certain area every season. I, for one, am qualified to judge all three levels of HT's, but have only had one invite in the last 3 years (unfortunately had to decline due to a work conflict).
I started running field trials 4 or 5 years ago and judging minor stakes. I know many hunt test clubs prefer judges with higher pass rates, resulting in more ribbons and happier participants, so I have to wonder if the perception is that those who cross over to field trials are no longer good hunt test judges and would be too tough. I would like to think that's not the case. I understand the rules and differences in each of the games and if anything, running and judging trials has broadened my experience level. Conversely, I know a number of hunt test judges that borrowed someone else's dog to run one time in order to qualify to judge at the next higher level. Some of those folks are asked to judge frequently. I just don't get it. Maybe it's just because I don't run many hunt tests anymore.... who knows.