If they had the same title that's probably what happened. It treated them as one email, so it only went to the first one you sent it to.
You can send one email to several people at a time, just by adding their names in the 'to' line. But you have to select each name as it pops up from the system and it will appear in the line with a comma following. Can't remember if it works if you do this manually or not.
