I had the same issue with January RPi firmware. Using 2018-10-11 with no issues.
duh forgot 921 pulls more juice.
mine pulls 1800 to mine at 19th
With Voltage offset to 0 mine pulls around 2200 with 21 TH/s
With -1 it is around 2000 with 19-20 TH/s and with -2 pulls 1800 and 18-19 TH/s