Here is a list of things you can do to troubleshoot why your document is not allowing you to insert or delete section breaks before or after a particular page:

1. Check to see if section break and/or page break settings are globally applied throughout the entire document.
2. Check to see if the document is set to follow a specific section break template you may have created.
3. Check for any hidden text that might be limiting the insertion of section breaks before or after a page.
4. Check for any specific formatting that could be conflicting with the insertion/deletion of section breaks.
5. Check your page arrangement and margins to make sure they are set to your preference.