Are there any tips and tricks on using the Qantas multi-city search?

I have been accruing Qantas FF points for some time now and had planned to use them to book a trip to Europe next July. This is my first time using points and from my research I had been using the multi city search on Qantas to find flights.
I don’t have a lot of flexibility with my dates, I couldn’t find anything suitable for MEL to Florence, so purchased flights.
I am now searching return flights. I had considered coming home via LA, using the multi city search (London - LA, LA-MEL) I could find a direct flight from London to LA with British Airways, but then could only see China Airlines flights via Taipei from LA to Melb.
When I did a single search for LA to Melb (on the multi city search) I could find a direct Qantas flight, but when I go back and try and put the two flights together in the multi search, the LA to Melb flight doesn’t come up as option.
Can someone explain why this is? I feel I may be missing out on flights by not searching correctly