It seems the commenting system sucks on here.
It might be true that you cannot reply to a reply, so you may have to reply to the original post I think.
And since it looks like you could be replying to the wrong comment, I highly suggest to use "@" symbol by any user-name or guest names when you are replying to a reply.
Wow, that was a interesting theory!"
That way, people can know who you are talking to specifically.
So that's my tip! I suggest to try to use it to make things easier.