Hello jetset1234 and welcome to Flyertalk.

I presume by 'Hong Kong Airlines and BA' there is a connection between the two carriers in Hong Kong? The fact that it's across BA and another carrier is probably the reason why the discount is not offered.

Perhaps try to price up a standard flight to Hong Kong and see if the discount is offered then? Obviously then if you priced up a separate Hong Kong-Bali flight you would have to consider whether there is a cost increase/decrease to splitting the individual flights, and the issue of flight protection across different tickets is also a factor.