Why aren't dynamic folders being generated for assets?
Problem solved thanks to Craft Twitter support; needed to re-build the Asset Indexes; because I'd added that field after already creating an entry the asset index was stale and needed flushing.


Open Craft admin dashboard
Click Settings
Click Update Asset Indexes
Click Go!


This process scans through the list of assets in the database and ensures it matches what is in the file system and updates the database as needed.