If you need to restore any of your purchased Notion Instruments, you will want to go to "Sounds" on the Home page of the app and then "Store." Here you can tap the button that reads "Restore Previous Purchases." You can then move over to the "Sounds" tab and view the status of the download. You must be logged into the same Apple account that you used to make your original purchase.
If you have a problem with an individual instrument playing back, you will want to go into "Sounds" on the Home page of the app and locate the sound. You can swipe to the left and remove the instrument. Once you have removed the instrument, you will want to reboot your iPad. You must reboot or you will end up loading the same problematic instrument that has been cached in the app. After rebooting, you can go back into "Sounds" and tap the icon of the cloud, to the right of the instrument, to download the instrument again. If the instrument has a check-mark to the right of it, it has been downloaded. If there is an icon of a cloud, the sound has not been downloaded.
If you have a problem with multiple sounds not playing back, you may want to back up your scores, remove the app and re-install. In order to avoid the download from being interrupted when downloading the sounds, you will want to disable the auto-lock function. This is done in the Device Settings for your iPad under "Display & Brightness."
If the problem persists, you will want to remove the app, reboot your iPad, and try the download once more from a different internet connection.