Hi Meo,
The free Classic Machines drum kits should be immediately accessible once you’ve restored your purchases in the shop, via pressing the similar named “Restore Purchased” button.
The App Store is fully handled by Apple.
Please make sure to log-in with the same account you’ve used when purchasing the Classic Machines in-app purchase.
In addition it might be helpful to log-off from the App Store, shut down the iPad and try again.
If it doesn’t help to solve the issue, please browse the web for articles how to solve App Store issues.
Please let me know this helps to solve the problem.
Best wishes,
Lars