There is a bug in this extension. This bug occures when a droppable div has the following css:

overflow:auto

and its content is longer than maximum height of the div (div becomes scrollable).

So when I scroll the div to the bottom, the scrolled part does not accept draggables. I tried to set the hover class with dark background for this droppable div and I found out, that the background is changed only in the part which was visible at the beginning before scrolling the div.