Have you done the required specific settings for email notification on risk detections?
If you did it and it works,if yes it is possible to something noticed kind of delay of not receiving some individuals alert if there are too many risk event occurring in a short period of time, when it is the case to only on notification is generated, that aggregate all of them.
You should also check on the risk level you specified for the notification because you won’t have notification for risk level lower than the specified level.
The links below may be helpful as well.