I suspect it's all been caused by the latest new feature that LJ is in the process of introducing, which currently permanent members are able to 'test drive'. The 'Known Issues' with this are increasing all the time, and one of them is that there are major problems with the email delivery of the notifications. Clearly that is having an impact on comments too.
So if I don't answer something immediately, or miss it entirely, you'll know why. ::Sigh::