First of all how do you have your R/C receiver connected to your Pixhawk?
Are you using the R/C transmitter Elevator (Pitch) joystick for the Rover Throttle control?
Usually the RCMAP parameters are ROLL = 1, PITCH = 2, THROTTLE = 3 and YAW = 4.
However, when I use a Spektrum R/C Satellite Receiver with my Pixhawk I have the RCMAP setup this way:
ROLL = 1, PITCH = 3, THROTTLE = 2 and YAW = 4.