[Mailman-Users] Only unmoderated messages getting to list?

Today we've noticed that messages we've approved aren't getting sent out by the list, or at least they aren't being received by list members.

I sent a couple of test messages to the list, and they got sent out ok. What could be happening that only allows unmoderated messages through?

There haven't been a lot of messages involved yet. Maybe three moderated messages have gone astray, from three different members, one of whom is a moderator.

We're on v2.1.23.