You gotta make sure both switches are on.

Jen writes:

My house is a two-story structure, the entryway being situated in between the upstairs and downstairs. When I first moved into this house, I knew the upstairs lights worked, but sometimes when I flipped the switch, nothing came on. It was a 25-year-old house, so I worried maybe the wiring was faulty. I had a feeling, though, that I had to figure out the right combination of on & off positions for the light switches. I finally figured it out: For some reason, the living room overhead lights (upstairs), the entry way light, and the downstairs hallway light are all on the same switch. Actually, the same two switches. One switch is in the downstairs hallway and one is at the top of the stairs up to the living room. They both have to be on for the lights to be on. So, if I turn the downstairs light off when leaving the house I have to go back downstairs at night to turn it back on before the upstairs lights will come on when I turn on the upstairs switch. Can't you just feel the frustration?