power lock problem?

Hi , I can not lock my 2000 chevy express v8 5.7L using power lock switch.I can open it but can't lock. The switch on passenger side works fine. I also can use remote to lock and unlock successfully. but can't lock using the driver side switch.Any idea, what's wrong?

Can you get the switch out of the driver's door and check the terminals with a test light while you activate the switch to see if it's working correctly?