Code makes no sense.
When you change the brand, it calls the SAME FUNCTION that changing the subcat does.
And that function *WIPES OUT* all content in the brand drop down!!!
It then RE-CREATES the brand drop down based on the selection in the subcat drop down.
So *OF COURSE* you can't select a brand!! Every time you try to, you are resetting the brand drop down completely.
As your code stands, I don't understand why you have an ONCHANGE handler for the brand dropdown, at all.
Try just getting rid of it!
<select name="add_prodBrand" />