Symptoms

After deploying a new version of a workflow, the notifications from the previous version ceased to send.
 

Diagnoses

This is a result of a lack of versioning in notifications. All versions of the workflow will use the most recent version of the notifications. In this particular case, a data field was added to the notification that did not exist in the previous version. As a result, the instances of the previous version of the workflow had no context for the data field and did not send the notification.
 

Resolution

You can work around this with the process instance migration tool available on the K2 Community site as this will move the process instances to a process version that has context for the data field.

You can also try removing the offending data field from the notification and redeploy the process. The notifications will no longer use the data field, and should send successfully - however this will affect all instances of the workflow, and none will use the data field.