In my experience stud finders are touchy. Try it on a known area where there is a stud. Then move it back and forth to get the light to go on and off. Then apply the same technique to the area in question.
finnegan made a good suggestion. Also, stud finders will find pipes and electrical wires, which I found out through experience. Drilled through a wall into what I thought would be a stud. Instead, it was a water supply pipe, installed (incorrectly) by the builder tight against the drywall. Caused a geyser of water to shoot across the width of my laundry room to the opposite wall. Had to run downstairs at top speed to shut the water off. Hilarious in retrospect, but wasn't real funny at the time.