Why won't my Stanley Fatmax Stud Sensor find studs?

Ensure proper detection conditions: check good battery power, use on drywall not concrete/block, place flat against wall parallel to door/window, slowly move while pressing center button, verify wall thickness is less than .75" (19mm) or 1.5" (38mm) depending on model. Smooth surface and within specified temp/humidity range. (Page 23)