Motors not running at full power

I am using pixhawk hardware with Ardurover stack, Skid-steering is enabled , I have individually calibrated my 120 Amp ESC as well During calibration each motor consume 6 amp. But, while i have connected it to pixhawk at channel 1 & 3 (Skid-Steering enabled), By using clamp meter i found that At full throttle both motor together consume 8 amp and in a matter of few seconds it reduces to 6 amp.
The MOT_THR_max is also set to 100.
How can I get the motors to operate at full power ?

A log and parameter file will make it easier for people to help you