Were you using the latest version? Anyway, it can be hard to tell what prefixes are on a worm-type enemy because only the head actually has the prefix, so you have to be hovering over the head (which is hard to do when it's always moving) and not just one of the body segments, which will just display the normal name. They're actually considered to be different NPCs that share a health bar, which is a little weird. If this is a big issue, I could remove the exceptions for worm-type heads and just leave all worms without prefixes.