Thanks, this is helpful. Given that the website is returning 'no flights available' for any dates I search, and I have to go through an agent, am I right that I should be asking them if they are able to "switch into the website booking office" (is that the standard terminology, do you know)? My guess is a front-line agent will have no idea what I'm talking about but possibly if I say the right things they will bump me to someone else. I know when I had a similar problem before the (experienced) agent couldn't do it directly, she had to call some other AC office (but I can't remember the name).

Good to know re expert flyer. I suspect something is broken with the ticket because if the explanation is that there is no availability in my (dual) fare class, wouldn't the website return flights but price them higher, corresponding to a move from T to B? It also seems odd that I bought this just last month but can find no flights on any dates I search between today and next November.

Also, I had confirmed eupgrades on this flight, and the agent had no problem removing them, which requires space to be available in the original booking class, which suggests that on at least that flight the underlying fare class was available. That said, I'm not sure whether removing an upgrade on a dual class fare requires availability in both fare classes. The agent just mentioned needing availability in B, but I don't know if the process is that she looks to see whether there is B available and does it manually, or if a computer on her end does it (which would make it more likely it checks both B and T).