Inconsistent pwm response from h bridge

Hey guys,

For the past few months I've been developing a digital cascade controller with a maple mini (ARM Cortex - M3) and a couple of pololu 37d geared motor with built in encoder. I've developped a good response controller using a L298n IC without touching the ground though. Now I've been trying to implement the same piece of code on two bigger h bridges: Monster shield, and Dagu Rover 5 board. In both cases the response to a signal of PWM is not linear. If I set the value of 80% dutty cycle for instance I'll receive almost the same output voltage ("filtering" with a cheap multimeter) of the 90% signal! So now is impossible to retrieve the relationship of VoltsxPWM and then PWMxMotor speed. Does anyone have any idea how to deal with this problem? 

Thanks in advance