I see a lot of "!TSF" and "NACK" which seems to be radio communication problems. That's MySensors related and can be related to many things, from soldering problem to power / capacitor issues. In MY_DEBUG mode you should see your temp/hum printed on serial every ~15s so there is something wrong there, either wiring or a code issue. Ty with the example included with the library "i2c_SI7021.h", without any MySensors related stuff, to see if your sensor is detected. You should see temp/hum on the serial.