Should be an easy one, I'm sure this was recently covered but I can't find anything from searching.

Want to check some prices on the AC site for some H class fares, however when I get the flight details, the Economy Lowest tab only shows N and L classes, whereas the Economy Flexible tab only shows Y and M.

Does this mean H fares are not available, or do I need to do something to get the site to show these classses?