With regard to the Reply button; this is the same situation we had on the memorabilia thread. Copied below is the response then and it basically is the same today. Seems it is a server issue that we do not control. This is the explanation as it was presented several months ago:
At the risk of getting too techical, here is the situation with regard to the
"REPLY" button. If you notice, when the reply button disappears, it is always on long, many-responses threads. This is how it was explained to us. Although we have discovered the reason for the problem, the fix is not easy!! Here is what we have been told:
Regarding the reply buttons, good news is we found the problem. Bad news is there isn't a fix for it that I'm aware of. The problem is the maximum length for a URL is 450 characters. I know you're going to say well there is only 60 or 70 characters in the address bar. That is true although the URL stored in the database is much different.
For example this is what you'd see in the database
If you get into the technical part of it each set of numbers between the / and / is a post ID. So every time a user replies to a post another ID gets added. For instance if you had 5 replies in a message and 4 people replied to the original message and 1 replied to one of the other replies the code would look like this:
/00000001/ (initial post by Joe)
/00000001/00000002/ (reply to Joe by Gary)
/00000001/00000002/00000003/ (reply to Gary by Joe)
/00000001/00000002/00000003/00000004 (reply to Joe by Gary)
/00000003/00000005/ (reply to Joe from John)
As you can see it adds up fast. The maximum number of replies to a single post is 49 (which if you ask me is a pretty lengthy conversation!). Again, I'm looking into where the limitation resides (Kentico, Internet standard, server standard or otherwise) and will let you know once I find out if anything else can be done.
I apologize for the problems with this. My suggestion would be to do as you are
already doing and find another reply and reply to it or just start a new thread.
Hope this helps a little!!