Symptoms
A workflow should email a message to a user when a task is assigned to the user. The task is assigned correctly it can be seen in the worklist. However, the email is not going out. Other email messages in the workflow are being sent (one as a user task and several as direct Send Email steps in the workflow).
Diagnoses
An issue with the 'Also Notify" field. Likely a null/invalid value getting passed in and the resulting exception preventing email delivery.
Resolution
Write an expression to more gracefully handles null values, and/or sanitizes white space, as detailed here: http://community.k2.com/t5/K2-Appit/IF-Expression-building-EmptyString-and-missing-Is-Blank/m-p/80434_M161