Robot Motors Drifting?

[FalloutSoftware]

Hey guys quick question, my team has a robot with 8in mecanum wheels. We have one Cim motor per wheel. The front right and back left wheels are slowly moving when it should be still. The question I have is: Is there a way to "calibrate" the joystick to the motors? We are using a brand new Xbox 360 controller. The is no drift in the stick itself because the drift is present while using the Attack 3 joystick as well. Please help me!! Thankyou!