When inserting a Nintex web part the "My Workflow Tasks" and "Workflow I've Started" may not be available for selection in the "Nintex Workflow 2013" web part category.

Reactivate the "Nintex Workflow 2013 Web Parts" feature from the site collection features. 
1. Navigate to the top level site settings > Site Collection Administration > Site collection features 
2. Deactivate Nintex Workflow 2013 Web Parts 
3. Activate Nintex Workflow 2013 Web Parts
 

The following 4 web parts are available in the category of "Nintex Workflow 2013"
- My Workflow Tasks
- Workflow Chart Viewer
- Workflow Report Viewer
- Workflow I've Started