The cause in all three cases was the same, so it is likely to be yours too. What probably happened is that, for some reason, your e-mail account registered something from LJ as spam or as excluded. If it does that, it will go on cutting out mail from it permanently. Go into your e-mail account and locate the list of excluded e-mail addresses, however it's called. Read it through; it's quite possible that LJ is not the only thing that was excluded and that you would not want excluded.
So far as I know, that account does not have a spam filter. I am looking at it now, but I don't find one. (I did find some interesting things stuck in the spam filter of my main account...before I recalled that LJ didn't have that email address.;-)
Quite a few people suddenly stopped getting notifications, me among them. Which is one reason I'm so bad about answering comments.