Try deleting the app entirely and reinstalling it. (This will delete the issues you've downloaded so far, so you'll need to re-download them. But you won't be charged again for issues you've already paid for). To delete the app:
Tap and hold down our app icon on your iPad screen until it starts to shake.
Tap the "X" in the top left corner, and then tap "Delete".
Return to the iTunes store and reinstall our app. If you've already bought issues, tap the "Buy" button and re-install them for free.