What type of switch?? If it's a hydraulic switch in the brake line, they're known to have a problem like this one. I think the diaphragm is failing. Most guys have now gone to the mechanical switch for this reason. Others don't think the hydraulic switch causes any concerns... Like Surfer suggested, try bleeding the system first. If it doesn't fix the problem, replace the hydraulic switch with a new one and re-bleed.

If you've already got a mechanical switch, have someone step on the pedal while you watch it's operation and see if there is something binding or in mis-alignment.. A test light or meter will tell you when the connection is being made and may help in diagnoses..