An option in the downloads preferences to limit the number of connections to a single remote host. Check it and another option lets you set a number, default being 3. This is for those hosts that will not let you get more than 3 files at a time, and the others all end up awaiting sources. With this option if you find four files on the same host, it will get three of them and the fourth will stay queued. Then when one of the first three download it will try to get the fourth, and it will end up downloading instead of awaiting sources.