How can I improve motion detection at night?

I've recently realized that one of my 3 cameras just isn't detecting motion after dark. It works during the day. I've fiddled with all the settings; if I turn sensitivity up higher, during the day I get detection on every leaf blowing in the wind. I can use live view, and see the area in the dark, but motion detection is very spotty. Sometimes I can stand a few feet in front of the camera and wave my arms, and get nothing. This camera is the oldest of the 3, a first generation stick-up. I think it used to work better, but I'm not sure. Do the newer cameras work better in the dark? Could I add an IR light, or do something else to improve detection?