publish_post suddenly stopped working?

I’ve been using HookPress to integrate with another system. For many weeks it was fine, and then suddenly publish_post is no longer working. I can’t see where my blog is sending the POST request at all.

But publish_page continues to work. I can’t find related system changes that might have caused a problem, I didn’t update WordPress or my operating system beyond some small unrelated tools installed. (I’m running Ubuntu 14.04.) I’ve tried watching the outgoing traffic with tcpdump, and I don’t see anything when there should be a POST to the other system.

I’ve tried the official HookPress version, as well as several updated forks from github. They all behave the same. I tried reverting to an earlier version of WordPress, which I had updated but before this started. A different blog on WordPress.com behaves as expected.

Has anyone seen a problem like this? Any thoughts would be much appreciated. Thanks.