When you say 'no FFB' and 'linear resistance', do you mean that there is something going on, but not as much as you expected? Or does it feel the same as if you've removed the power? (ie, I can unplug the mains from my wheel (DFGT) and it still works as a controller, just with no FFB or lights. There's a tiny bit of mechanical resistance left, which is linear).
Remember also that the castor you have in your setup has an effect on the force being supplied to the wheel. Put it up as far as it will go and drive in tight circles to get LFS supplying the wheel with a big value for the FFB. Or drive straight and fast with no hands on the wheel to see if it starts a shimmy.