Oh I don't think it's just one bad apple. Call me negative, but I think within agencies it is bushels of bad apples. The bigger and more successful the agency, the more likely the apple will go bad. Just too many moving parts and each of them awash with cash. Writers are easy targets since the payments are often slow and can become so complicated (regions, subsidiary rights, foreign rights, film rights, translations, audio versions, etc.) and a writer's attention is often on their next project.
Negotiating for split payments from publishers sounds like a minimum starting point. We need to ask a LOT more questions of our agents. It sounds like the agents need to ask a lot more questions of their financial people as well.