I was just rummaging through my PMs and noticed a few single emails that weren't (?) replied to. At least, when I clicked the "Show conversation history" button. So I'm sorry to those who I have not replied to in the past! I never meant to give anybody the cold shoulder!
You might have responded to them and the other person might have deleted the response. As things go right now if the other person deletes your response you won't be able to see it. Also, you can't see responses ahead of that mail, for example if you reply to a mail you won't see that in the history unless you look at the history of their reply to that reply.