previously purchased audiobooks not showing up

I recently purchased a new iMac, iPad 2 and iPhone 4S in the last 6 months. During that time, I have lost soem audiobooks that I purchased through synching errors. When I attempt to navigate to the "Purchased" section of iTunes, it doesn't display that I have purchased any audiobooks. I know that iTunes has a record of my purchases because I have tried to re-download an audiobook that I purchased and it prompt's me with, "You have already purchased this audiobook, buy again?"

Any ideas on how I can get access to my purchased audiobooks?

Thank you.