Speed brakes don't deploy

On my B-737, I use the autobrake knob and arm the wing spoiler. This is supposed to brake the plane automatically upon landing. The wheel brakes actuate and the wing spoilers flip up. This lets you concentrate on landing the plane. You arm the wing brakes by pressing [Shift] + [/]. Problem is, sometimes the wing brakes work, and other times they don't! I view an instant replay of the landing from "spot plane" view and I can see that they aren't up. Other times they are. This once caused me to run out of runway at LaGuardia and the plane went into the river! It was a short runway - 7000 feet, so there was no room for error. If I am doing something wrong, I don't know what it is. Or else it's just another of the numerous glitches in FS 2002.