Motor Shield R3

Hi everyone.

I have a problem with the arduino motor shield rev3.

I connected it to an Arduino UNO without stacking it, I use cables from the outputs. I did this because I need to control 6 motors and I have 3 arduino motor shields, if I stack all of them they will not work.

I am using 9v batteries, one to power on the UNO and another for the motor shield.

The problem I have is that motor shield output A is running slower than output B, but it only happens in the forward direction, in backward direction both outputs speeds is the same.

Any suggestion?

Thanks