Ed, You should be getting emails from Trulia letting you know that your listings are expiring and that you can renew them. Lookout for those emails and this will prevent your listings from disappearing on Trulia.
You have to keep your listings updated. Go to My Listings and under each listing Status click on Refresh and it will add days to how long your listing is posted. If you don't Refresh it to extend the number of days, it will disappear. Hope this helps.
Try contacting Trulia through the "help" link. Posting a question under the "Advice" section probably isn't going to get to the tech support folks. Also, keep in mind that this may have to do with your own company's MLS feed. You may want to contact your company's web/marketing people. Good luck.