I called UA yesterday to book a revenue ticket CDG SLC TLV - which wasn't bookable on the website too. Had a long discussion with the agent (who needed help from the fare desk), as to why the rules have become so complicated that no-one knows them - they can just try and then see what the computer says.

When something fails to price, or to ticket, or to revalidate - they often just have to guess what the problem is and try to fix it. Fundamentally, since the actual decision rules are hard coded in non-human readable form, the computer has no way to easily explain what the problem is. It's not just a poor computer system, but a problem with they way fares and rules are done in the industry today.