That can happen if any of the Moderators delete a post.
Posts are deleted if they contravene Forum Policy, and such instances are spam, eg advertising, illegal, eg offering software that are pirated copies, and other reasons.
Sorry but I can't say why those posts you get alerts for do not show, but my guess is they were spam posts.
I've been getting help with a virus problem through the forum and this has been on going help, but the last two email alerts yielded nothing from the link when I opened it. Any ideas?