It all depends on the settings for the SMTP server, some hosts setup the servers so that any mail bounced back that originated from a script on a site gets deleted so you don't get an inbox full of bounced back emails.
The emails will be sent if they have selected the option, so it will be problems with SMTP srevers, either your hosts or theirs.
It's also worth checking, but my guess will be those people who are not getting the email notification are using either Hotmail, Yahoo, etc. free email accounts, have spam filters in place, or have exceeded the amount of mail allowed on the web server.