You may also have an issue if your phone's storage is full. In that case, you'll need to delete photos/videos from your phone.
For iOS devices:The most common issue is that the WeddingMix app has not been given permission to access your camera roll. Go to SETTINGS >> WeddingMix. Make sure Photos, Microphone, and Camera looks liek the picture below.
For Android devices:
Swipe down on the top right of your screen. Look for the "Rotation Locked" button. Tap to turn off. (For more in depth info go to: howtogeek )