Contacts who are active in the Workflow and have not yet reached the new action step will be scheduled for it after their current action executes.
For example, your workflow has two actions, and the contact is scheduled for the second action. Now, let's say you add a third action. Once the contact executes the second action, the contact will be scheduled for the third action. Contacts who have already passed the previous action and completed the workflow will not complete the new action.
Since actions are scheduled as soon as the previous one executes, if you want to ensure that your new action executes for all active contacts, you will want to add it after the action that the earliest enrolled active contact is scheduled for (you can check the History or look at your actions to see how many contacts are at each step in the workflow). Alternatively, you could add the new action as the last one of the workflow, assuming no contacts have completed the workflow yet.
For more on workflow timing changes and other troubleshooting tips, check out this article from the Workflows User Guide.