That is the expedcted behaviour in the case that you left your luggage for longer than 6 hours. This is clearly comunicated before you lock up the luggage. The lockers of course don’t open before the payment has been made in order to ensure the user does acutally pay.
If you did not occupy the locker for more than 6 hours and it still asked you to pay more, that would be a grave malfunctionment of the locker and you should contact SBB via the contact center: 0848 44 66 88 (0.08 CHF/minute, waiting times may occur).