What is the purpose of M1 and M2 (connected to victor 5 and victor 6) ?
Are they two motors connected to the same gearbox ? If that's the case, then arcade drive would run one of the motors forward and the other backwards - unless if you have one of the motor outputs from one of the victors wired in reverse, the motors will be running against each other in the gear box...
You can also check the output led on victors 5 and 6 also - changes in the led color and flash rate can help debug the motor drive system...
http://wpilib.screenstepslive.com/s/...tem-components