It's not usually good value for money (typically just 0.7p per Avios) so it's only worth doing if you have no other use for the Avios, but the main reasons they don't show are for trips which involve some BA non oneworld partners, for tickets with complex or special fares, and also when your BAEC login has dropped off. So if you login cleanly and then book a straightforward trip it should come up. There was a glitch a year or so back where using Incognito was sometimes needed, but hopefully not now.