Hi,
Notion isn't supposed to download purchased instruments each time you load the app.
Either the downloads were not finished the first time you tried to download them, either the app isn't properly installed.
Please check everything is up to date (your iOS device and your Notion app) and try downloading again. If nothing improves, uninstall Notion (but save your creation first !) and download the app again from the apple store.
If, in spite of it all your problem still persists, I'm afraid you must open a ticket with the support team...