Why are alert emails delayed or never sent?

Why are alert emails delayed or never sent?

We have an alert configured to send an email for any group membership changes of several groups configured on several domains. Sometimes a group is modified but the tool doesn't send an alert email. Usually the change is logged in the list of Active Alerts.

Most recently we had several group changes and no emails were sent until the following morning when a large number of emails came through well after the changes had been made.

I'm wondering if there's a known interval of time which, if exceeded, the event is not considered current enough to sent an email, but it will still be logged as an alert. Is anyone aware of anything like this?

Is there some kind of tuning option so if the event happens and ADAudit Plus doesn't read the event information for x minutes after the event, it can still alert on it?