Frangatang: I wouldn't say 'numerous.' We had fifteen bookings where the aircraft didn't arrive, but at least ten of these phoned in advance to cancel (including two who phoned later to say they had to turn back). So there were just a handful of slots which were not used.
The broad slot system is aimed at spreading out arrivals and it does a good job. Inevitably, though, there is some bunching and some quieter spells. If you arrived at a quieter time, it was probably for this reason, rather than due to no-shows,
Alan.