A direct debit fails. For example because of insufficient funds or an old account number. Stripe then retries a few times, spread over the following days.
Your member gets an email. With a link to the payment portal, where they pay themselves or update their payment details. In the app, their subscription shows that something is outstanding, with the same button.
You see it. The arrears appear in the money overview and in the member’s record.
After a number of days, their access is paused. By default after fourteen days; you set it under Account & profile → Payouts and invoices, between 1 and 90 days. They can then no longer book or get in, and get an email about it.
Once they pay, it opens up again by itself. You don’t need to do anything.
Want to chase it yourself? Send them a message from their record. A personal message often works better than a third reminder.