Motorshield Problems

Hey Guys,
i have a big problem with the Arduino Motor Shield R3, i hope you can help.
Im using it to control a little Car with an Iphone. In one Direktion everythin is fine, but wenn i change the direktion to drive backwards the motor just get's less than half power. I just used the basic scetch.
Maybe somebody had the same Problem like me and can help.
Thx Josh

I just used the basic scetch.

That isn't posting your code. that's a vague description. Post your code or a link to this "basic scetch"
so we know what code you have.