Workflows that are not set to run immediately will be added to a Queue. By default, the queue processes 15 items every 5 minutes so actual run times will vary slightly. This batch size can be changed in settings.
When are Events Created?
It’s important to note that queue events are only created based on the Run Actions workflow option. If this is set to anything other than Immediately a queued event will be created. This means that certain triggers like Abandoned Cart, Subscription Before Renewal and User Has Not Purchased For Period will not appear in the queue even though they are triggering based on a schedule. If you can’t find a workflow which has been run, try checking your Log.
A queued event can fail because its missing a data item. E.g. it supposed to email to a certain user but that user has been deleted. When this happens, it remains in the Queue Events table for 30 days and is marked as failed. From the Queue Events table you can attempt to run the event again, but it will not reattempt to run by itself.