Can someone point to any posts or other info that explains what (for pure AS itineraries) currently can and can't be booked online using the AS website's multicity award search function?

"Just try it and if it doesn't work, call to book" doesn't answer the question. I'm looking for info on the website's restrictions and logic.

For example, why does it find and permit multicity online award bookings for SEA-JNU (stopover)-ANC but will error out on attempts to do the same using KTN, SIT, or WRG for the stopover in the Southeast? The errors occur despite award inventory being available on the routes/days in question.