First, Run it on Arduino. If it is ok, then convert that code according your MCU.
@legoguy has already done that and reported success further up in this thread
@johnyrobot5 has already mentioned that it works on Arduino.
That’s the whole puzzle:
- It’s not the hardware (HTU21D breakout board, Particle device)
- It’s not the software (system firmware, application code)
- What can it be? The wiring? The combination with the power shield (I guess this was checked already)?