Your Proof pixel needs to be installed on all Capture Data and Display on-site urls in order to use the Recent Activity and Hot Streaks (conversions) notifications. Without the pixel, your campaign may fail to capture conversions or display notifications.
How to check your campaign
Click on the relevant campaign > Scroll down to 'Capture & Display'
Now click on all of your Capture Data and Display on-site urls + confirm that the pixel is present on each page.
Can't find the pixel on a page?
If you've stumbled across a Capture or Display url in your campaign without the Proof pixel installed, then you'll want to get the pixel installed before you do anything else.