The scheduled data source or import set definition has been manually deactivated in the Data Source configuration. (high likelihood)
The 'Run as' user specified in the scheduled job is locked, inactive, or has insufficient permissions to execute the import. (high likelihood)
Another import job for the same data source is currently running and blocking the scheduled execution in the import queue. (medium likelihood)
The import set table or transform map associated with the scheduled job has been deleted or corrupted. (medium likelihood)
System maintenance mode or import processing has been globally disabled via system properties. (low likelihood)