If you look upthread you will see my lamenting how hard it is to book more than 4 on a single PNR and that often when tools like EF show I bucket with 4 available you can only book 3 on AS.com. There seems to be an n-1 kind of availability on Alaska.com but that is not picked up until they try to process payment. My guess is that is what you are running into, I had the same thing. You need to use EF to find I=n+1 (so I=2 if you are trying to book solo) for it to proceed and issue. It does work in the end though, it is not general phantom availability but more confusion in the logic of AS.com about just how many are available.