iTunes 11 with library on external HD won't resume downloads

I am running iTunes 11 with the media files stored on an external hard-drive. This setup works great except for one thing:

If I am currently downloading something from the iTunes Store and I quit iTunes (either pausing the download first or not), when I reopen iTunes and it "Checks for Available Downloads" it can't find the interrupted download - I have to go back into my purchases to start the download again.

However, the temporary download files are appearing as expected in the Downloads directory on the external hard-drive but iTunes doesn't appear to be seeing them.

How can I make iTunes see the interrupted download and not have to restart the download from the beginning?