I was exasperated by my Wahoo KICKR performing faultlessly in the evening, but consistently failing during the day.
Every time I would set up for an afternoon ride, with sunshine streaming through the window, the KICKR would always read 0W. There was no Bluetooth connectivity issue, and the fault was constant on both Wahoo Fitness IOS app, and with Zwift on a variety of platforms. I tried them all.
Following advice from Wahoo, it turns out that the optical sensor can be “blinded” by sunlight, and will fail to read the rotor properly.
I made a sunshield for the sensor out of electrical tape, and now everything is working fine.
Get a piece of electrical tape and fold it lengthwise with sticky side in. Then, get a second piece of tape and stick the first piece to it lengthwise along one edge. Then, stick the second piece of tape along the top of the KICKR, with the side with the folded over first piece of tape shielding the gap between the rotor and the base (where the optical sensor is located).