Redmine

Hi everyone,
I got a problem with mail notification that I configured on my RedMine project:
I've my project with differents modules and for any module I've some users mail configurated, for example:

Project 1 - module A, B and C - module A with users Jim, Jhon and Jack, module B with users Jim, Jack and Chris and module C with Jack, Geronimo and Martha.

Sometimes not everyone in same group receive correctly all mails when an user open a ticket or when we update a ticket.
Why is that happening?

Thank you for your help!