It is best to use CircleCI's Slack Orb to easily integrate custom Slack notifications into your CircleCI projects.
The Orb allows users to create custom alert messages for any job or receive status updates.
There are examples of code snippets that can be used as part of a workflow and all of the code that makes up the orb can be found at the bottom of the page.
Note: There are old integrations of Slack that are no longer supported by CircleCI. This is often time why users are not receiving Slack Notifications.