First, make sure the pwms are actually connected to the motors you think they are.
If you find the IFI dashboard application, you can connect your computer to the Dashboard port on the OI and look at the pwm outputs directly.
Next, I would suggest using "Find in Files" and searching for any other occurances of pwm13, pwm15, or pwm16.
Also I don't know what Exponential_Grow contains, but you could check there as well.
If all else fails, try temporarily replacing your drive code with something simpler and reintroducing parts of it until you find the problem. You could start with something as simple as this:
Code:
pwm13 = p1_x;
pwm15 = p1_y;
pwm16 = 255-p1_y;
It may also be a good idea to disconnect the motors and just watch the outputs using the dashboard until you find the problem.