Now christep, no need to state the obvious for us Diamond members, but perhaps we can shed a little light for others.

It looks like the flight that you want to get on is full in Business Class. That, you know , can happen. Business Class is full, that's about it. On the other hand, CX must hold at least a few Business Class (Booking class:J) for its Diamond members for their 24 hour seat guarantee.

The only thing you can do is be waitlisted and check periodically if any last minute cancellations occur. Note that CX is infamous for letting waitlist reservations remain on waitlist while they release some seats that are available on general inventory. ie - you'd have to get your travel agent to check periodically!