If those subscribers were added to the sequence via automation and now showing as "completed," they will no longer receive the new additional emails on your sequence.
If you would like to place them back so they would receive the email sequence, you may create a temporary tag, add that tag as an additional entry point to your automation, and add the tag to the subscribers you want to be pulled into that automation. After that, wait for those subscribers to be pulled into your automation; then, you can delete the tag and entry point. They should receive the new email sequence.
How to Manually Add Subscribers to an Automation10
And if you are planning to add more emails to that exact sequence in the future, I suggest setting the email sequence to "Yes, subscribers added via Visual Automations should stay in the sequence."
If you're having some trouble with this, I recommend you email email@example.com to take a look at your account.