Hi, not sure why you're having that problem, since I'm also using a 6S and everything works fine here.
Holding the power button until you get the power off screen, then holding the home button till you get to the home screen helps clear the memory.
I do that when something gets stuck.
Other than that I don't know what else you can do, except delete and reinstall the app.
I don't think you'll lose your progress, since you register a username when you post your score.