Dear Paula Whipple,
Good day! Thank you for posting in Microsoft Community.
From description, when you record each page separately you cannot replace a narration on a specific page.
May I know if my understanding of the scenario is consistent with yours?
If yes, I tried to test in PowerPoint from our side. I can replace a narration on a specific slide with steps below:
Select the slide, click Record tab > click Record > Clear > choose "Clear Narration on Current Slide". Then click Record again, chose "From Current Slide", record a new narration for the slide.
Then I exported the presentation to MP4 file, the narration worked on each slide.


I'd like to help you, we can work together and try to narrow down and resolve the situation.
Before moving forward, could you confirm information below? It would be very helpful for the troubleshooting.
1.How did you replace narration on separate slide and record narration again? Could you share your detailed steps?
2.Does the issue happen in a specific PowerPoint presentation? If you create a new presentation, does it work?
3.What is the version of your PowerPoint app? We'd like to test in same version and see if we can reproduce the issue. Please go to File>Account, capture a screenshot of Product Information, as shown below.

Look forward to your reply. Thanks for your cooperation!
Hope you keep safe and healthy always.
Sincerely,
Tina | Microsoft Community Moderator