PowerTap cadence is accurate within 40 rpm and 130 rpm. Anything outside of this range will be recorded as a zero. Since the cadence measurement is a virtual cadence (based off of torque measurements) and not measured with an extra sensor it cannot read values outside of this range.
If you're using the cadence separate cadence sensor, make sure the sensor is paired to the computer and that magnets are lined up properly. Magnet and sensor should be about 2mm away from each other when they pass.