DW doesn't EMAIL you if crossposting fails. They put a message in your DW inbox. So if a cross-post fails, go here to see if there's an explanation.
http://www.dreamwidth.org/inbox/For me it said
Failed to crosspost entry to entropy_house@LiveJournal: Incorrect time value: You have an entry which was posted at 2012-01-12 16:17, but you're trying to post an entry before this. Please check the date and time of both entries. If the other entry is set in the future on purpose, edit that entry to use the "Date Out of Order" option. Otherwise, use the "Date Out of Order" option for this entry instead.Which... my brain not working too well today, I took to mean that if you start making a post on DW, but don't actually post it until after you made a post on LJ, it won't crosspost the DW which HAS to go at the end, but CAN'T go at the end because it has an earlier time on it.... don't take this as actually correct or anything... I am not hitting on all cylinders today.