This could have multiple causes. In most cases, people just made a mistake with a setting or left a button off.

Most common causes:

The "on"-toggle was still left off for the domain and/or individual notification
The Proofly-pixel isn't installed on the desired page
The URL is not matching the one in the filters
The domain is registered without turning "Include subdomains" on
The user reached it's monthly impression limit
The notification doesn't match any of the filters
The notification simply doesn't have anything to show (yet)