Trouble sending out Mass Emails/Newsletter

Ello, I'm a staff member on a forumotion board and recently we've had a problem with our mass emails not getting sent out properly. Some users would get them and others would not yet in the options to send it out, the option 'all members' is selected. I was wondering what might be the problem with this.

Some members may have chosen to opt-out of having newsletters sent. Each member have this option in there preferences.