I have a fixed size div (roughly 800px square) with overflow set to scroll. This contained div contains a few dozen smaller sortable divs (approx 200px square).

All of the divs that appear on the screen without scrolling work fine (I can drop them onto each other no problem), but anything below that point (seen once you scroll down) will drag fine, but cannot be dropped onto.

This occurs with regular droppables too, not just sortables.