Members not recieving e-mails

I was wondering if anyone could help me out, a few of my members are not recieving e-mails for new posts in a thread. They have ticked the box in the options for 'notify me when a reply is posted'

any way of fixing this?!

Have they checked it in their spam mail?

yes, they recieve some e-mails and not others, sorry i should have said that

You sure they watch that topic?

yes they say they've had some beofre from the topic and its just stopped now!

Does it happen to you as well?

Profile>Preferences
Display a notification when new replies are written :Yes

Have they done that?