Maybe it's me, but it seems for a while now it's impossible to reply to a youtube comment by clciking Reply and then the message has the G+ name of the person your replying to, ie :
+JohnSmith Yes what a great cat video!
Now you can only reply to the thread as a whole.
I've tried it in G+ within YT, YT on it's own and also in the main G+ screen that lists all comments, no joy.
Am i being blind?
Cheers, Gary.