Scott is correct that you'll have more control over these listings if you submit them to Trulia manually via the link below. However, if you don't want to go that route, you can contact our Customer Service team via http://www.trulia.com/help/ask/ for assistance. They can examine the feed that provides your listing information to us to locate the source of the problem.
The most common cause of a listing expiring within your account is that it's been classified as stale because the listing information hasn't been updated or changed in quite some time. In that case, you can make a simple change to the listing, like update the listing description, and that should cause it to display on Trulia as active once again.