The same happened to me... it can happen if you kill Samuel too close to the southern wall then the wand can drop in the locked room. What I did was save the game when I had Samuel close to death - then I didn't have to replay as much of the fight if something went wrong. Then, before finishing him off, I lured him into the center of the room. Then the wand should drop where you can reach it.
- Telemachos
