This happens occasionally when the iPad interrupts the processing of the issue. Typically, restarting the app will resolve the issue. If not, you can always delete and reinstall the app. You will never have to pay again for something you've already purchased, as long as you're on the same iTunes account. On reinstall, you'll see a box asking if you wish to restore all purchases—click “Yes.” If you miss that box, just tap “Missing Issues?” in the Library tab to get back access to all previously purchased issues and subscriptions.