I'm not sure what you mean by "not processing". If it's a workflow definition not being triggered on a condition or schedule, you could check the Workflow Admin Console, under the Monitoring node, in Subscribers to check how your starting rules are being evaluated. If the workflow definition is being triggered, you can use the Workflow Designer and search for instances of that definition to see which activities failed to complete and whatnot.

For workflows that fail to complete, there's a new feature in Workflow 9.1 called Workflow Web that lets you view Workflow information such as definitions and instances. You can enable a notification feature that would send email to administrators in the event a workflow is terminated. You can configure terminated notification settings for each workflow definition or use the Workflow Admin Console to set up a notification timer that would periodically email a list of terminated workflow instances since the last notification.