It's important to try and add all of the actions in your campaign before activating it to prevent users completing the campaign before new actions are added.
The reason for this is because once someone joins the campaign they get enrolled in that version. If you add messages to a campaign once a user has joined they will not get them.
If you do add messages or amend the campaign, any new users will see the updated campaign though!
Users are marked as complete once they have completed all actions on the campaign. For example if you have 3 emails due to go out in a week and you wait until these have all been sent before adding a new action to the campaign, the system will have already recognised those users as completing the campaign and will not send out any additional actions that are scheduled after.
You can track how many users in your target audience have completed the campaign by looking at the main screen on your campaign as below:
If you want to add another action to a campaign and can see your target audience showing as completed as above, it's better to create a new campaign with your additional actions. To do this:
Duplicate the campaign
Remove all the previous actions
Add the new action
Make sure all of your actions match the campaign type (smart scheduling is a relative feature only, not fixed or immediate)
Activate the campaign