Your Proof pixel needs to be installed on all Display urls in order to display any type of Proof notification on your website.
How to check your campaign
Click on the relevant campaign > Scroll down to 'Capture & Display'
Now click on all of your Display urls + confirm that the pixel is present on each page.
Can't find the pixel on a page?
If you've stumbled across a Display url in your campaign without the Proof pixel installed, then you'll want to get the pixel installed before you do anything else.