Happens to me more frequently than I'd like(3 times this year), often due to missed connections / rescheduled flights.

What I was told initially, and has worked well, is go to aircanada.com > Customer Support > Customer Relations Send Us an Email > Complaint > Air Canada Altitude and then fill in the rest from there.

Aeroplan gave me the run around at first saying AC needs to fix it, so that's what I ended up doing and it's worked 3/3 times within a couple days miles are posted