Forum

I think that "mail now" is a bit of misnomer... As far as I know, this means that instead of respecting the forum-specific delay for mailing a given post, it is added to the mail queue right away. The mail queue will be processed at the next cron run, whenever that occurs. That means that there can still be a delay until the mail actually goes out. For example, if your forum is set to allow editing of posts for 20 mins and cron runs every 5 mins, "mail now" means that your mail will go out within 5 mins instead of within 25 mins. Caveat: there were some substantial changes with regards to cron in the recent versions of Moodle, which may affect this, but probably not or not significantly.

If the understanding is not the issue, there may be a technical problem with the scheduling of mail-now mails on your site. There have been a few threads about different technical issues. Search the forum.