I looked at the code and it seems that if one wheel is turning the other should too, codewise. I think you have an electrical problem or perhaps a bad servo.

Try swapping the left and right servo connectors to see if it's the actual servo that doesnt work. If the one that doesn't work now start working when you swap places it's either something in the code or a bad connection on the circuitboard. Do you have 6V and GND on both servo connectors?

It can also be a bad pin on the PIC, can you try moving the servo control lines from PortC.0 and PortC.1 to PortB.0 and PortB.1 for example. Don't forget to change the code accordingly.

/Henrik Olsson.