There is an FAQ on the TPMS system, it's a little hard to read right now as we are still working on some issues with our new forum software:
faq-tire-pressure-monitoring-system-tpms-t330539.html
Bottom line is, the TPMS system can be very picky. I would try using a different handheld tire pressure gauge to see if the pressure is correct.
Do you live at high altitude or travel to work at a different altitude? TPMS sensors are altitude sensitive as they read absolute pressure (it's not referenced to air pressure outside the tire). Those who live at higher altitudes find they have to bump the pressure up to keep the TPMS warning light from coming on. Air pressure drops about 1psi for every 2000ft in elevation. At sea level the tire pressure can be at 35psi and will drop to 30psi at 10,000ft.
If tire pressure is not the cause, then there may be an issue with the TPMS system itself. The only way to start diagnosing it is with a scan tool.