Distribution Groups certain individuals will not receive emails within the DL gourp

We currently use Hosted email (exchange online service) the issue i am experiencing is certain users within a Distribution Group will not receive any email from that group.

Steps Generally done to Add or Remove users in DL or groups:

1.Click Manage under Exchange on the Admin page
2.Click Distribution Groups under Users & Groups
3.Click Details when selecting the Distribution group’s display name
4.Choose “Add or Remove” under Membership

Also, if i remove and add that users multiple times within the Distribution group, the users will receive email from that group periodically then eventually emails stop.

Any ideas?