What is the protocol when it's not possible for Apple to charge the monthly subscription? If the user gets denied access to the App, what is the standard procedure for the following months? - Does Apple try to charge money again or not?
Monthly subscriptions - procedure when not possible to charge?
If the user fails to renew an auto-renewable subscription then they are not asked a second time. They need to request a new subscription.
When a user is denied an iTunes purchase, say for not being able to connect to the store, invalid credit card, etc., the burden is on them to correct the issue(s) and try their purchase again. The store's only response will be to alert to that initial billing/connection/account failure.