Text box not appearing in a Messages conversation

I have a recurring problem in Messages on my Mac where, in one group conversation in particular, the text box occasionally disappears. Instead of having a text box at the bottom of the screen I can type into, the conversation simply goes all the way down. When this happens, the text box still appears in every other conversation; I can also still send messages in that conversation from my iPhone. It disappears only on my Mac, and only on that one conversation. The problem persists when I shut down Messages and start it back up. Sometimes the text box comes back, and then later disappears again, on a pattern that I have not been able to discern.

Any thoughts?

Messages 8.0

Mac OS 10.10.2