I had the same exact problem outlined in this and other threads on the first summoning test. My hang up was I had two spider legs--one from act 4 and one from one of the previous acts. I left the room, dropped everything, picked up the ingredients and rune, went back in, no luck. I went back out, dropped the one spider leg I was carrying and then picked up the other one I had. Went back in and that did the trick. It seems the program is looking for the leg you get in act 4 which is impossible to tell without trial and error since it looks exactly like the leg from the previous acts.
Hope this helps any other pack rats who hold on to every unique item they pick up!
