Yea, LAX-HKG is tough to score op-ups, and MUCH harder than HKG-LAX (a route that seems extremely op-ups friendly, as Cathay Boy points out. I've scored one, maybe two J-F op-ups on LAX-HKG out heaps of flights over the last 5 years, and anecdotally it seems much harder compared to SFO. I have come to believe the reason is CX has two midnight flights ex-LAX, leaving within 1:45 hours of one another (CX881 and CX883). CX check-in staff at LAX have perfected the art of balancing loads on both those flights perfectly, so unless both are oversold on the same day, op-up is unlikely.

Why is it relatively harder to get op-upped LAX-HKG vs. HKG-LAX? Here's an example that illustrates how this (theory of mine) works:

1.) Say CX883 (11:50pm departure) is oversold in Y with empty J seats, while CX881 (1:30am departure) is oversold in J but has a few spare Y seats. Both flights have a few seats in F open.

*Result: No op-ups, both flights go out nearly full. CX will convince early-arriving CX883 Y pax to switch to CX881 (maybe saying "we'll give you an aisle seat" to pax who previously had the middle. And many pax will take this, because both flights still arrive HKG in the wee early hours). Meanwhile, they'll convince early-arriving CX881 J pax to swap into the spare J seats on CX883. I've had them offer this to me a few times.

2.) Now say ex-JFK flights, CX841 (10:05am departure) and CX831 (2:55pm departure), the two back-to-back ex-JFK departures. Same loading scenario as the LAX example.

*Result: Op-ups would happen on both flights. The first flight pax would be op-upped Y-J, and the second flight pax would be op-upped J-F. Primary reason is just the check-in time for the two flights is much more spread out than the two ex-LAX flights, so they can't use the second flight to rebalance loads with the first.

So I think ex-LAX is harder than other places because CX has two flights leaving at nearly identical times, and they re-balance pax to reduce op-ups as much as possible.