Build a segment of customers who have not booked for a while and look at it once a month. It is the cheapest new work available to you.
Why lapsed customers get missed
Nobody cancels. They just stop coming and because there is no moment where it happens you never notice. A customer who came every six weeks for two years and has not been in since March is invisible unless you go looking.
Choosing the window
Base it on how often people normally come. For a six-weekly haircut, three months is lapsed. For an annual service, three months is nothing. Set the window against your own pattern.
What to do about it
Contact them. Not with a discount straight away, just a note that it has been a while. Plenty of people meant to rebook and forgot. That costs you nothing to fix.
Why they left
Some moved away, some found somebody else, some had a bad visit nobody heard about. The last group is worth knowing about and they will usually tell you if asked directly.
Common questions
How far back should I go?
Two or three times your normal booking interval. Beyond that they have genuinely gone.
Can I contact them if they never opted into marketing?
Not with marketing. Contacting somebody about work you have done for them is different. The line is worth respecting.
How often should I check?
Monthly is plenty. It is a slow-moving list.