I tried so many steps to get this to work correctly & I have most of the content now, every so often I get something new installed so I'm not sure if it is something I'm doing or it is their servers. I'll list what I have done thus far and the last thing I have done has been working but mind you it might be coincidence that it has worked & that will be true if it is something with their servers, It's worth putting up anyway to see if it helps though.
I tried uninstalling and reinstalling fresh installs and deleting the garageband related folders located in private/var/folders/
That seemed to have done nothing by itself, tried that numerous times.
I tried redownloadign the content over and over never worked.
Cleared my app cache, app logs and everything else that I could possibly delete that dealt with any system applications, that didn't work either.
Then I enabled the debug menu for the App Store and reset the application, I don't know if this actually worked or the files just decided to install.
It seems to have worked twice but never does it install the full content.
The first time I got the drummers installed but not the Liverpool, Manchester, or Motown Revisted kits but I just did it again and it installed the Manchester Kit.
I still need the Liverpool & Motown Revisited, currently trying to get those installed but like I said this could be coincidence.
To enable the debug menu open your terminal and enter
defaults write com.apple.appstore ShowDebugMenu -bool true
Next open the App Store and click on the Debug menu, next choose the option Reset Application.
Again, I don't know if this actually worked or if it was just lucky coincidence so give it a shot.
This definitely did not help the download internet issue, that is something that is definitely with their servers. This might have helped with the issue of everything downloading fine but the content never showing up, I did have to download the content again for the missing kits but twice now something has installed (keeping my fingers crossed and trying the rest). Their servers seem to be bugging out again so I don't know how long this will take.