Instruments Randomly Missing (Audio file x.aif not found)

Hi,

I'm using Garageband v.10.0.3 on a MacBook Pro with Yosemite 10.10.3.

I have previously downloaded and installed all the additional content from the GB menu.

Today I purchased MainStage 3 in order to download the legacy JamPack content and have done this by ticking the appropriate boxes in the download management box.

In my instruments menu in GB I can see all the software instruments installed, both the Garageband factory sounds and the legacy Jam Packs.

What I'm experiencing is that some instruments work and others don't. For example, on the top Bass menu, the first 5 selections work fine until I get to 'Prog Rock Bass', then GB comes up with a box saying 'EX instrument Prog Rock Bass.exs: Audio file "IL Rick Bass Bomb 03.aif" not found!', and asks me if I want to search again or ignore. If I click search again the box just keeps coming up. It keeps doing this for other instruments completely randomly.

I'm assuming that perhaps not all the files were correctly installed (?) but I haven't moved anything myself. Everything is where the installers placed them (Library/Application Support/GarageBand/Instrument Library/ so i'm trying to understand how I rectify.

Can anyone help?

Thanks in advance,

Neil