Alaska Airlines' website is also useful, since they show connections which don't show up elsewhere. Right now you can get BOS-SEA-LAS for 20K Avios in Coach one-way (12,500+7,500), which doesn't show up on aa.com.

You're up against a few whammies for that date. It's Valentine's Day, it's a Saturday (timeshare swap day), and it's mid-winter, when people in Boston are looking to head somewhere warmer.