Why can't I reply to YouTube comments?

I know there are a lot of these posted but I haven't seen the same problem I'm having yet so I don't think this is a double post, apologies if it is. This isn't a problem with old comments, I know they don't work any more, but even with a comment I posted just last night. I get an e-mail saying someone replied and I click the link to the YouTube comment section containing the reply and my original comment. However, instead of the comment box appearing when I click the 'reply' button, all I get is a long rectangular box that I can't click into or type into and the 'reply' button is faded out and cannot be pressed. This happens to some comments it seems, but not all. I've also tried navigating to the comment without the link in the e-mail by searching YouTube for the videos name and locating the comment that way but I get the same box so I don't think it's a problem with the link in the e-mail. Thanks in advance for any help.