What happened to some of the replies on comments on YouTube?

I've been searching online. Some say about Google+ but these are RECENT comments from 6months ago. Some 1yr to 5months to 11/12 months ago SHOW NO reply button. It's simply not there at all. Some users don't have reply button to reply to their comments. Do you think they turned it off? They have an account Google gmail + to comment on YT, but ONLY some people don't have that while others who commented 1yr to 5months ago HAVE reply buttons for us to reply to them. However, it isn't always the case and is miscellaneous, out of order, random. Is it a glitch? I tried on Google Chrome to reply using my own account, but some replies don't show up. I tried Safari. Other browsers and same problem. Help? It's not my comments but others, and I'm sure my comments YEARS ago don't have functional reply buttons.