If you made employees as leavers and they aren't appearing on the Full Payment Submission (FPS) for the period in which they left, this might indicate they have been marked as previously submitted as leavers.
When you process leavers, a prompt appears asking whether you've already submitted the leaver details to HMRC.
If you click Yes, because you've confirmed HMRC already have the details, your next FPS won't include them. You can tell if this has occurred by clicking on the actions cog for the employee, and if there is an option to Unmark as a leaver, it means the employee was marked as already submitted leaver.
If you meant to do this, there's nothing else you need to do. If you've done this by accident, to resolve this, follow the steps below.
Click on the actions cog against the employee and select Unmark as a leaver.
Next, mark the employee as leaving again making sure you click NO when prompted.
Click Save.