It is very difficult to say, why some do receive the email and others not.
As long as there is no error message from the mail server that you are sending the mails to, it is almost impossible to figure out what could cause the problems. Obvioulsy Limesurvey is sending mails out.
Question is, what happens to the other emails? Are they sent or not?
Do they reach the opposite mail server?
Are they rejected by the mail server? Are they rejected by the user?
So you see, there is a lot of stuff that can go wrong.
Which method do you use to sent the emails?
Is there a way to check of any email bounced back?