I am trying to book a simple MEL-HKG-MEL itinerary with one small hitch: I want to travel premium eco flex on the outbound and eco flex on the return (travelling with family on the way back only).

You can book this online, because when you search for eco fares, it shows you premium eco as well. But when I do this, it shows me that my preferred outbound flight is sold out in premium eco. However, if I start a search for premium economy fares, it has premium economy (W class) still available for that same outbound flight. Of course, that's no good because then it doesn't give you the option to book a lower class for the return.

Calling the service centre brought the same response, no premium economy left on my preferred outbound flight.

The only explanation I can think of is that you can't combine W class premium economy with K class economy? But this doesn't make any sense - I'd be happy to pay more to get on my preferred outbound flight, why wouldn't Cathay just take my money?