It is possible you have started using the app with only a single device, and later decided to buy a second one. If this is the case you will need to go the profile menu shown as the second item on the bottom of the app.
On this screen you can press the settings button.
Select the Left or Right LifeEar, depending on which side you have recently bought.
Next click on the “Setup Left/Right Device” button and follow the instructions in the App.