What is your GarageBand version and your MacOS X version? How did you originally buy GarageBand? Did it come preinstalled?
In the current version, GarageBand 10.0, you activate the missing loops and instrument by by buying them, see: Expanding GarageBand for Mac (v10.0) content with an in-app purchase
In the previous version, GarageBand '11, the loops and instruments should be installed by "Software Update". Is your GarageBand application installed in the applications folder and simply called "GarageBand"? If not, put it back there. Sign into the AppStore with your AppleID, and reload the Software Update tab by pressing command-R.
I am not sure, if it is still possible to download the missing GarageBand '11 content, since there is now a newer version. If you succeed, tell us please.