Try booking on their website, any stopover or multi-city itineraries via HK, and if either the outbound or return linked legs don’t have the same fare booking class (I.e. M, M, for outbound and V, V for return available), or the connecting/transit (hours in/via HK) rules not met, you’ll get en error message saying technical difficulties when you either proceed to seat selection of try to select a different flight. You then have to call them to book the same itinerary as their internal system shows which fare booking class is available and sometimes they have to do a manual override to allow your preferred transit/connection hours (or days) in/via HK.