Q: Garageband Lessons on iMac and MacBook

I've just started using Garageband to learn the basic of playing the guitar and bought the additional lessons while using my rMBP.  This morning I opened Garageband on my iMac expecting to find the lessons there (all other purchased content has been available on both) but get the option to buy the extra content again.  The 'Restore Purchases' option in the menu is greyed out, and remains so even after I've entered my Apple ID into Garageband (still only get the option to buy the extra content).

Is there a way of getting Garageband to recognise I've already bought it or do I have to purchase it all again for the iMac?