Automation sequences can be very complicated things, which is why it is important to map out your process before you build the sequence. However, once built the contacts should get each of the steps, however, there are times when they will not: -

1. If they unsubscribe/complain or bounce from your emails, they will not be able to get any in the future including the rest of the campaign

2. If they are deleted/archived from the system, this means that they are no longer on your system

3. If they are added to another campaign which then takes them out, part of the automation allows you to take people out of one campaign. This could be what has happened to them

If you are still worried about people not getting all of the steps, please raise a support ticket