The Box – File metadata update event will not properly start the workflow if the Connection used to configure the start event was created prior to 1 December 2017. While the workflow did publish and is listed in the Workflow list, the start event does not register and hence when the event takes place, the workflow will not start.
 

Create a new connection to Box and use that newly created connection when configuring the start event.

This does not affect any existing workflows with existing Box start events (Nintex Workflow for Box, New file, New folder).