The bank is only above water at certain tides, so the boat leaves when the sea says, not when the timetable would prefer.
People notice that our sandbank departures are at nine one week and half past ten the next, and assume it is disorganisation. It is the opposite.
The bank we use is a strip of sand about two hundred metres long and it is above water for roughly five hours around low tide. Outside that window there is nothing there at all, just a pale patch you can see from the boat. So the departure time moves with the tide, which shifts by about fifty minutes a day.
That is also why we will not simply move you to another day at short notice if you turn up late. The boat is not waiting on us, it is waiting on the water, and it has to be back before the bank goes under with people still on it.
The practical advice is this: when you book, note the time on the confirmation rather than remembering what time your neighbour went. And be at the jetty ten minutes early. On a small island that is easy, and it is the difference between a good morning and standing on the jetty.