First try to restart the app, and close other apps that might be running in the background on your device. If your device is running iOS 7 or newer, double tap the home button then swipe up on an app to close it.
On Apple devices running iOS 6 or earlier, you can do this by pressing the home button twice then holding your finger on one of the apps that will pop up, and finally hitting the red circle that will appear in the corner.
You can also try restarting your iPhone or iPad, this can help to clear up some basic app issues.
As a last resort, you can try to uninstall and reinstall the app. Redownloading the app is free once you've paid for it once, so don't worry about any fees.
If you're still stuck and having an issue, please email us at firstname.lastname@example.org and let us know what device you're using, which Dr. Panda title you're having an issue with, and what sort of issue you have. Once we know the problem we can try and work on a fix, if we aren't already.
All of your feedback is valuable to us, so don't hesitate to send us an email and let us know!