VEX EDR issue with motor not working

My kids are programming their robot for the BEST robotics competition using the EDR hardware. We are using basic 12 volt motors. When I straight wire and test the motors both work perfect. When Using the sample codes for single or dual joystick controls from the sample codes one motor will not work and the other only moves very slowly. They are configured into the correct ports and all wire are connected correctly. Any help for my kids would be appreciated.

Are you using the 29 motor controllers

The voltage for motors is 7.2 volts, so a 12 volt motor will run slower