Where did you test it? On live site or in the preview?
check if it works in the admin interface (so you don't have any other custom html, script or styling involved):
Does it not respond to a change, or do you get wrong/unexpected data?
Any errors? Check for script errors in your browser, or perhaps in the Kentico Event log?
Make sure the naming of the field is correct when referencing the parent value from the child dropdown!