Try this and let me know what happens.
Disconnect all motors
Do "pwm 1:50 2:50"
Do the leds near the motor connection light up?
then do "pwm 1:-50 2:-50"
Do the leds light up the other color?
If this part looks like it works then attache the motors and try it again and let me know exactly what the leds do.
Also, what kind of battery are you using? Voltage, capacity, type, ect.
Ringo
ok, maybe I understand your problem now. When you say they do not move at PWM value of 50, will they move at higher values? If so then nothing is wrong. I've never paid attention to exactly where the threshold is where the motor starts. PWM is simply a percentage of the time the motor is turned on. At a PWM value of 50 the motor is on 50% of the time and off 50% of the time. Every motor has a threshold where it will get enough power to start turning. It depends on the efficiency of the motor and the voltage applied. So if you took a little Radio shack motor rated at 1.5V it would probably turn on at a pwm value of 10. Likewise if you increase the voltage from 9.6V to 12V your Traxter motors would start at maybe 40 instead of 50. So if your motors turn slowly at pwm of 60 or 70 then that is fine. They should go pretty well at 99 or 100. Please let me know if this is the case.
Every motor you buy is unique, so the 4 you have will probably start at lightly different values. This is why it is so hard to make a robot drive in a straight line. That is exactly why we have included all the PID functionality into the serializer. while 1 motor may go faster than another even if both have a PWM value of 70, once you connect the encoders and use the MOGO command the motor ll travel at the same speed. The MOGO command issues a PWM command, then checks the speed, modifies the PWM, checks the speed, ........ in order to keep the motors at the speed you want, even with motor variation.
I hope this helps, please let me know.