Are you searching for that pair directly? As there is no BOS-LAS direct service on Oneworld carriers you might want to try finding the intermediate legs yourself before then calling to book. Ba.com doesn't deal with internal USA availability very well.

On AA.com you can check for award availability on their routes. Probably BOS-DFW-, BOS-ORD-, and BOS-LAX- are the most likely ones. On US some gateways are PHL, CHT, PHX. Don't know if you can see awards on the US Air site, haven't looked before.

Hope that helps.