The website is always full of flaws and errors.
The current Exec Class seat sale isn't bookable online for a lot of routes. Maybe because it has been loaded as a mandatory roundtrip fare with a Saturday night stay and within NA the website only looks at oneway fares.
Works great on the GDS and via 3rd party sites... but ac.com doesn't price it out and the sale ends today. YUL-LGA on SABRE is $940.00, AC.com $2670.00.
And they still promise that their lowest fares are always available online?
Why Lise Fournel still has a job I will never understand.