5681 SFP works, VI's do not.

[JohnFenner]

There is no way to ask the device if it needs to be zeroed.  If the temperature drifts too far from the temperature at which it was zeroed, it will return an error from Read or Fetch and the LED on the device will turn yellow.

Taken directly from the "NI RF Power Meters Help":

Note You must zero the NI 5681 before you take power measurements, particularly when the device operates within the lower 20 dB dynamic range of the device. Zero the NI 5681 every time you connect it to a USB port. If you make frequent low-power measurements, check the sensor zeroing often and repeat as necessary.