So I've done some searching and can't seem to find an answer to this---why are Air Canada flights not showing up as an option for purchase when I run a search via UAL.com?

I am trying to book a trip from Kansas City (MCI) to Copenhagen (CPH) and several weeks ago when I ran a search via my phone one option that showed up included Air Canada flight 882 (a direct Toronto to Copenhagen flight) paired with a direct flight out of Kansas City to Toronto (YYZ).

Searching today on my desktop (need to go back and rerun the search on my phone), I can't seem to come up with anything including AC882. I have tried changing dates, changing my route (forcing it to simply search Toronto to Copenhagen), changing class of service etc. Basically, I've tried everything possible to get UAL.com to show AC882. It just doesn't--I've confirmed seats are available by running the search on Air Canada's website--multiple days, multiple classes etc and there is availability across the board on this flight across multiple days. Yet, UAL doesn't show it.

What am I missing? Is there any way to force the search engine to use a particular flight?

Thanks for any advice--I'd really prefer to fly two legs between here and Toronto vs the 3-4 UAL.com seems to want to force upon me.