Nodes never expire

I've configured my feed importer to expire nodes after 3 months, and I've double-checked that the published date is being properly set on the nodes, but nodes that are much older than 3 months (4 years!) never expire. When I create an entirely new source node with the feed importer and the initial import runs, it imports all of the available content, included everything older than 3 months. If I then run cron, the nodes are still not deleted. Completely stumped.

There is another thread about nodes not expiring; the initial poster closed the thread but other people have left comments about the same problem.