Depending on the type of device you have you may need to do something called a “forced restart” with your device.
If you are playing on an iPhone of iPad that has a Home button at the bottom of the screen. This can be done by holding the Home button and the Power button down at the same time until the screen turns off. Keep holding the buttons down until you see the Apple logo.
Once the device turns back on, open the app again, you should see your newly unlocked characters.
If you have an iPhone X or a newer device that does not have a Home button you can force restart your device by pressing the volume up button, then releasing it, then pressing the volume down button and releasing it, followed by holding the Power button for a long time until the screen turns off and the Apple logo appears. Once you see the Apple logo you can release the power button and open the game to see if this has fixed the issue.