No shows (people do not show up for their reservation) have been becoming a serious problem recently, both with tourists from abroad and local customers. Many high-end sushi restaurants or Japanese restaurants are individually run and have the small number of seats (about 10) so that one owner can keep an eye on the whole place.
Repeated no shows can be a serious threat to the business of such restaurants. After you make a reservation, try not to cancel to the best of your ability. If you have to cancel, please contact the restaurant as soon as possible. The restaurant may charge a cancellation fee if it is close to the reservation, so check their policy when you book a table.