In two threads, 1 have the answer button and 1 doesn't? Picture in detail :
No Reply (Answer) button?
Ops, found out that the answer button is at the top of the page, but it should be at the end of last post for convenience 
Well, there is a "quote" button in the each post. The "answer" button is for the whole thread and that is why it is located at the start of the thread.
And at the end if the thread is more than 5 posts long. But generally there is no difference between "quote" and "answer".
Do you think that this button should stay at the bottom even on short threads?
Very clear sir! 
I'm testing and it is amazing!
I think the Answer button is not really well placed actually. No matter the number of answer already in thread, there should be an Answer button after the last post.
In my opinion, the Answer shoutbox should even be present in any case after the last post. No need to open a new page, right ? And take the risk for the user to forget what he wanted to say.
I am agree. And implementing it is technically easy - it is a matter of some html/js in the templates. Actually in my opinion, the editing and posting new messages should be provided by one single user interface. Not very sure how it should look though. 