another session done and the inconsistent resistance is back

goign from 2.5 and leaving the peleton behind up to 5 w/kg and getting dropped even though the group was steady flat 2.8 w/kg pacer group .Odd thing was i was increasing gears to keep up on the hills .Very odd !
Only just noticed a video about trainer calibration via zwift which i didnt know was a thing , how easy is it to do ?