The below article is relating to the workflow builder which can be found in:

Administration > HR > Workflows

There may be times when you have a workflow that hasn't triggered for 1 or multiple employee's.

One of the reason for this can be down to the restrictions you can set for workflows relating to Departments, Sites, Companies, Business unit, Cost centres and Headcount classifications.

Brief Overview on restrictions

Essentially what these option do is give you the ability to limit who the workflow triggers for when published for example if you have a survey that only one department needs to fill out you can limit it to only trigger to all employees in that department.

Important thing to note these restriction wont stop you from publishing or setting the workflow for an employee, HRWize will just check afterwards wither the selected employee falls into the restrictions placed.

Example

An Situation when this issue might occur for you is if you have multiple offboarding workflows. 

If you have a situation where you have publish the offboarding workflow when making them leaver but the workflow does not trigger.

First place to  check would be what is set in the Departments, Sites, Companies and see if they are in the the one included in the workflow settings. if they are not either update the employees site or the workflows settings to include the new site and then republish the workflow to the employee.