+1 on expecting some esters and diacetyl in a Mild, but they must be in proper proportion.
Take one of your beers and let it warm up to room temp.
Then taste it with a critical eye, and with the BJCP Style guidelines open. (
http://www.bjcp.org/2008styles/style11.php#1a)
All off flavors will be accentuated. This may be what happened to your beer, it was late in the flight and therefore it might have warmed up before they got to judge it.
This is also one contributor to beers that score very highly and then go on to mini-BOS or BOS and then get knocked out quickly. There are many times I've judged beers in mini-BOS/BOS with blatantly clear off flavors, and wonder how the hell did THAT beer ever come into contention. (Yes, even with beers I advanced myself).
My point is that these off flavors don't necessarily show when the beer is cold, but bloom in full ugliness when the beers warm up. Use this to hone your process. If you can't taste/smell the off flavors when its cold, but they emerge when the beers warm up, then you haven't eliminated the root cause of the off flavor. Once you get them to smell and taste just as well when warm as when they are cold, they will NOT be effected by temperature changes during judging and they therefore will score better and stay in contention much longer during head to head competition phases like mini-BOS or BOS.
Like Drew said, try to spend some time with a ranked BJCP judge (or certified Cicerone). If you can't, then get ahold of the doctoring instructions in the BJCP exam study guide:
http://www.bjcp.org/study.php#drbeer(or better yet, do both together).
Volunteer to help with competitions near you and enroll in a BJCP exam preparation class so you can become a qualified judge. Then you can judge your own beers yourself and you won't need to rely on other people's feedback. That's how and why I first became a judge and now I'm one of the Assistant Directors for the BJCP.
HTH-