Jeff...

Not all properties on the multiple listing service are place on Trulia

Jeff,
All innernet web sites have limited uploads. Many are sponsored by individual local Real Estate companies or national franchises, others are sponsored by local MLS systems, or advertising mediums (ie Homes and Land), and some (like Trulia and Zillow) are 'independent'. Many sites, like mine at http://halere.com, have multiple feeds from several other sources.
I think that you will find that the most complete list is on REALTOR.com, the official site of the National Association of Realtors. You will get contact info for the listing office at each home listing there.
Hope that helps

Some agents might not input them to Trulia. Some companies might not have feeds that upload their listings to Trulia.