Hiya,
I ran into a few problems at first when using the scheduled listings. API error, usually and some would post and others wouldn't.
My errors usually consisted of, keyword spamming or forgetting to set a needed detail, postage/shipping, etc; or incorrect time/dates.
The usual method once I'm happy with a listing is to run it through Auctiva's error check at the bottom of the listing and if it passes that, then preview it to make sure I'm completely happy and then set the time and date to start it.
So far, touch wood, spit at the cat, it's working.
Hope it helps a bit,
K.