Select the slides with the problem and make sure they all have the PowerPoint’s “Slide number” and/or “Footer shapes”.
To do so, delete the shapes that are meant to act as Slide number and Footer shapes in the slides.
Insert the footers and page numbers using the “Header and Footers” button in the PowerPoint “Insert” toolbar:
Select the “Slide number” and/or “Footer” checkboxes and apply the changes.
If the slide number and/or Footer shapes don’t appear, make sure they are in the layout of the slides.
If they are not, change the layout of the slides and re-insert the “Slide number” and/or “Footer” shapes.