UIScrollBar component Question

At design time, If I attach a UIScrollBar component with a dynamic text field and pre-populate text into the text field and test the movie, works fine.

BUT… if I create an empty dynamic text field then attach a UIScrollbar. At run time, I load the text field from an external text file, The UIScollBar is greyed out,

why ??? Please advise.

By the way, I am using Flas MX Professional.

Thanks