Wondering if someone here has some theories on something I think it strange but maybe not so strange to the airlines.

I have flown what is no CX-865 maybe 15 times over the last 5 years, that is a flight from JFK to HKG with a stop in YVR. I though only take it to YVR and have done that each time. I was looking to fly this on Dec 2, I see there is no availability. Then I checked further and there was no availability from JFK to YVR on ANY day (in Dec 2018) for ANY class and that was even stranger. So I then check in Jan 2019, there is availability. So then I change my destination to HKG and the date I needed to fly (Dec 2) there is availability? So it seems like Cathay would be trying to fill more long haul passengers than folks like me getting off in YVR. I am then only guessing there are not enough folks buying tickets on the second leg (i.e. YVR to HKG). I called Cathay but the representative simply said my computer does not show availability. I don't think she was understanding my thinking that if there is room to fly to Hong Kong then there is room to only fly to Vancouver. Expert Flyer seems to indicate there are seats available in any cabin (I though am not a premium user to be able to search for actual availability)

So this posting / question is:
- anyone have suggestions on how to communicate this with Cathay, to see if they can open up 1 seat
- anyone else see something like this and was able to get anywhere?

Additional update:

I found this interesting a 1 way ticket JFK to HKG (coach) is $684.00 on Dec 2 2018. Compared to on Jan 6 or Jan 27 2019 (only 2 days I checked) JFK to YVR is $703. So its actually more expensive to fly well under 1/2 the distance. Financially Cathay is making more on the people flying only to Vancouver.