Is the Asiana leg really a codeshare, or just booked under the Asiana number?

In the latter case, aircanada.com won't show it. Phoning AC, they may or may not issue the ticket. OTOH, Asiana might. Or not.

I had two similar instances recently, one with AC and BR. Neither would issue me the ticket, and I ended up buying through a TA.

I also bought a ticket through a TA that involved AC and NH. I could not buy it on either web site. Admittedly in that case, I did not try phoning either airline. AC actually does codeshare on the NH leg, but would only price at full J.