Email group class does not receive the email

Our customers can choose from 3 groups, the customers of 2 groups namely puppies, and adolescents will receive email reminder, the customers of basic group will not receive email reminder. This invitation of the class is sent on 1 day before. We currently have 3 lesson groups , 2 groups the students are reminded. 1 group people do not get a reminder. The question is how is this possible? The group that does not get a reminder is "basic1"