Garageband loops are not downloading

Some of the songs I recorded using a previous version of GB used these loops, which are now unavailable.

Have you kept the previous version of GarageBand installed or did you uninstall it completely, including the loops? The new version does not include all loops you may have used with the previous version. You could try to restore them from an earlier backup.

Every time I open the software, it looks as if it is downloading content, but nothing seems to be updating.

Have you reindexed the Loop Library?

You can do that from the "Loops" drop-down menu (the last entry) in the Loop Browser.

If you have not tried it, start your Mac in Safe Mode, launch GarageBand, and perhaps the download will now install. SafeMode clears some caches and disables non-standard system extensions. See: Mac OS X: Starting up in Safe Mode

If nothing helps, try a variation of this tip and install the download manually:

https://chentiangemalc.wordpress.com/2012/06/28/case-of-the-continually-failing- garageband-software-update-macos/

• Quit Garageband and all other running applications.
• Open a Terminal window (the Terminal.app is in Applications > Utilities).
• Copy and paste this command into the Terminal:
  open $TMPDIR
  (This command will open a Finder window with temporary folders used by applications. This folder is hidden among the system folders. Be careful not to delete anything).
  
• In the window that opens scroll down to the folder com.apple.SoftwareUpdate. Look for a subfolder "content" and inside it "downloads". Is there a download waiting?
• If yes, delete the complete "com.apple.SoftwareUpdate" update folder. And for good measure the "com.apple.garageband10" folder in the $TMPDIR as well.
• Restart the computer and try again.

I have completed in-app purchases already and am signed into the App Store, so that is not the problem.

Still seems like a GB problem, though.

It may be an App Store problem, even if it used to work previously.

I just discovered, when I tried to download the "Red" loops for my iPad, that I had to reenter my credit card details on all my macs in the iTunes Store and the App Store, and on both the iPad and the iPhone. The reason, I had signed into iCloud from a different Mac the day before, and now Apple wanted a confirmation of the payment details on all my devices, before the in-app purchase for the additional loops worked. There was no indication at all of an Apple ID problem on any of the devices. I had to go to "View account" in iTunes to discover this.