Here is a list of things to consider to help troubleshoot the issue:

1. Check if PDF file is visible when opened with Acrobat or other PDF viewer.
2. Check if using the correct version of PowerPoint to support PDF insertion.
3. Check the resolution of the PDF file.
4. Try to insert the PDF as an image.
5. Check if the PowerPoint content is closed during the PDF insertion.
6. Save the presentation and try re-opening it.