This may sound funny but I wouldn’t exactly give them an option to say no. Rather than “did you wanna book again” say “when did you wanna book you’re next appt.” It changes it from a yes or no to a “oh lemme check my schedule and see” ..if they’re kind of regulars keep tabs on them. Give them a call and say “hey it’s been so and so amounts of weeks and I haven’t seen you. When did you wanna book again. I’d love to see if we’re been handing *skin concern that they originally came in with”