I have seen this problem randomly on my windows 7 computer. After close observation, I found that DRAG and DROP won’t work from low privileged window to High privileged window. I generally run a command prompt with administrator and try to drag drop items from my explorer to the elevated command prompt. Since the elevated command prompt is running in high privilege mode, and my explorer in low privilege mode, drap-drop from explorer to cmd is not working. Whereas same operation to a normal command prompt is working without any issues since it is in the same privilege mode as my explorer.
I hope this small briefing helps you come out of drap-drop dilemma.
Btw, command prompt appears like below when it is in high privilege mode.