I think I've tracked down the reason for missing posts in my LJ feed from my blog. Mine is for TV show reviews and the subject is always the name of the show, the ep title and the air date. There's two of us posting to the blog and we may both post about the same ep and we may post more than once about the same ep. Only the last entry on the ep is showing up in the LJ feed, the other posts are not there.
So the fix to keep edited posts from showing up twice may also have the effect of losing some posts? :(