why don't iTunes automatic downloads work on my iMac?

Whenever I select the Check for Automatic Downloads option in iTunes it always comesback with a message saying that all downloads are compltete on this computer, even when there are some TV series programmes to update. I then have to find the new episodes manually & click on download to force them to download. My iMac is authorised but still can't recognise that I have programmes to download. Help?