When the app is asking to make your In-App Purchase again or if you can't restore your previous purchases, it's usually because of the following:
- You're using multiple iTunes accounts
- You've bought the app with a different Apple ID
- You have 2 iPads with 2 different accounts but syncing to the same computer.
To avoid this, just sign-in using the first Account you bought the Noteshelf app with.
If you're not sure from which A/C you bought Noteshelf app with, you can you check your iTunes purchase history to see if Noteshelf and the in-app purchase is listed there. More info here: http://support.apple.com/kb/HT2727
If In-App purchases are not showing up properly, you just need to hit the "Restore" button in the in-app store to fix any issues in the storefront or with your previous purchases.