The most common reason is that you are holding your phone incorrectly.

The second most common for this is that Orientation Lock is activated on your phone. Turn it off.

For iOS devices:

You can see if Orientation Lock is on your iphone by swiping up. If the Orientation Lock button is highlighted just Tap to turn off.

Restart the WeddingMix app.

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 )