Why am I not getting any alerts in one of my notification integrations (Slack, Pagerduty)?
Alerts are only sent when a rule is triggered. For example, if the rule condition is when "an event is first seen", an alert will not be dispatched until a new issue is created. (Depending on how many unique issues your project is receiving, this can take some time.)
Ensure that the notification integration is enabled under the rule actions, either as the specific service, or covered under "all enabled services".