AC serves 5 ports in Cuba - and they serve VRA (Varadero) more often & from more cities in Canada than they do HAV. VRA is only a short bus or taxi ride from HAV so is quite often a viable alternate with more availability and options than HAV.

You may be finding it hard to get seats on AC at the time of year you are looking for as I believe mid-March is spring break and it's also Easter this year as well. After the winter, there are a lot of people heading from Canada to warmer climes for the break. (Always had trouble on AC's flights from SYD-YVR during March/April because of the amount of traffic heading to/from HNL, which was a stop on the old YVR-SYD route).