Sometimes apps do not work as they should be. There are many external reasons for that, so the main solution of restarting the app usually solves those kinds of bugs: 

At the Home screen of the iPhone, or while in an app, swipe up from the bottom of the screen and pause while still pressing the screen. When the App Switcher comes up, swipe left and right through the different app cards to find the app that you want to close. Use a quick swipe upwards to close the app.