Sometimes iPads don’t communicate very well with the Apple servers. Fully close the magazine, tap on the home button to ensure you have exited the magazine, and then double tap the home button. All the opened apps will appear at the bottom of the iPad. Press and hold on top of the magazine, the apps will start to shake, tap on the red minus sign to fully close the magazine.
Make sure you are logged in with the same Apple ID that subscribed to the magazine. To do this, tap on the Settings app, scroll down and tap on the store app, double check that the same Apple ID is being used. If not, sign out and sign in with the correct Apple ID.
Relaunch the Magazine. Go to the Newsstand app and tap on the magazine to launch it. The splash page will appear and then the magazine home screen will show. The page should be updated and now the yellow subscribe button would have been replaced with a grey Restore button.
Restore all purchases by pressing the restore button if desired to re-download all previously purchased magazine issues.