WP doesn't send any confirmation email to new users

As you can read in the title, if I add a new user to the blog and select the box to notify the password to the new user by email, new user doesn’t receive any email with password.
I have installed also WP-Memeber plugin and it worked very well until some weeks ago. Now, new user only receive an email when they first register on the site, but after activation from admin they don’t receive any email. The strange thing is that if they try to restore the password they get a new one. So the only email that is not sent is the email with password after activation by the admin.