I m porting code from QT3 to QT4

I have a widget with 2 frames, one with some drag-able icons, and another a ListView tree
In the past(QT3), I can drag the icon to the ListView tree. However after porting to QT4, I can't drag that!(seems the area of the ListView does not allow the icon to drag in)

Can anyone tell me why and how can I set that back???