Cannot download additional content for Garageband

Whenever i try to download the additional instruments and loops for garageband it always gets to the end of the download say something like "update could not be expanded and verified. It may have been corrupted or tampered with during downloading". Im so sick of trying to download 1.2 GB time and time again only to find it never works!

If anyone has any suggestions please tell me! I am using a macbook air from mid 2012 and do not have ilife on a disk!

Is it worth taking it in to an apple store?