I installed Garageband 10.0.3 on my MacBook Pro 2013 when it was running Mavericks. I then purchased the additional sounds. I delayed downloading them all because at the time I didn't need them right away. I installed Yosemite, and now want to download them, however when I go to GarageBand > Download More Sounds the option is grey and clicking on it does nothing. I tried signing into the Appstore and opening Garageband through there, and it didn't work. I've restarted Garageband several times but with no luck. I'm wondering if I need to reinstall it, however I'm not quite sure how to do this. If anyone knows how to fix this, please let me know. If you need more information or this question needs to be edited, please let me know. Thank you for your time.
The issue has been resolved try downloading again.
Source: Apple Support
I had a similar problem and contacted Apple Support. They told me that there seems to be a problem with a server and that I should try again in the next days.