Take this for what it's worth -and you may want to ask marainsanity since she has a MUCH better handle on video fandom- but I get the impression that AMV fandom and the people who run the contests at cons tend to suffer from trendiness. Certain series/effects/types of video are popular for a time, others aren't, and this may have something to do with why your AMVs didn't make the cut at those two conventions. They just weren't what those fans were looking for in terms of style and/or stock material.