Sometimes the Slack integration may not be pushing out alerts from Opsgenie to a customer's Slack account. 

Because of this, alert notifications won't be created in Slack. To remedy this, you would need to go to the Slack integration and click upon the "resyc" button which would be:

If you are having trouble receiving the create alert notifications, please resynchronise your integration.

Ideally there would be an automated system that would run checks on Slack to ensure that the notifications to Slack would be pushed out properly once alerts are created.