No online ticket after 2pm Hong Kong Airport to Zhongshan

Hi!
I am not able to select a ticket HKIA ferry after 2pm. F.e. on 14th May. It does not read "not available", it does simply not appear. Also not on other days.
Does anybody know the reason why I am not able to book tickets after 2pm? Is it risky to purchase a ticket at the Airport, meaning everything is sold out?
Thank you,
Alex