Looking at the page, you have the Main Cat select list and the Sub Cat select list.
You have applied the Populate List From Array behavior to the onChange event of the Sub Cat select list.
The Populate list from array behavior needs to be applied to the onChange event of the parent select list. It appears the Main Cat Select list should be the parent.
You should apply the Populate list from array behavior to the Main Cat select list instead of the sub cat select list.