If you're using Microsoft 365 Copilot Brand Kits, try creating a new file/presentation and reapplying the Brand Kit there. Also verify that the Brand Kit still contains the expected templates/assets and that those assets are accessible. Missing or unpublished templates can prevent the branding from being applied correctly.
If the issue started recently and affects all new content, it may be a service-related issue that requires further investigation. Please let us know which app you're using (PowerPoint, Copilot Chat, etc.) and whether you see any error messages.