On my iPad quitting an app and restarting it often helps correct problems like this. Here's how to do that on an iPad:
Go to the Home screen and double click the Home button. That will reveal the row of recently used apps at the bottom of the screen. Tap and hold on the app in question until it wiggles and displays a minus sign. Tap the minus sign to actually quit the app. Then tap anywhere on the screen above that bottom row to return the screen to normal. Then restart the app and see if it works normally.
Can you do this on an iPhone? Let me know.