I have built 6 variants of the same vehicle and it either hovers on the front end or doesn't hover at all. each side has three wheels spaced out evenly. All of the vehicles have been around 2000kg. Another question is should I just work toward a flying vehicle.
I once managed to reverse the direction of the hoverpads, so that they sucked my vehicle to the ground, instead of pushing it up.
Other than that, I can only think of an odd weight distribution, having the center of mass being way in the back of the vehicle, and thus overloading the rear pads. Try moving the rear pads a little more further back.
The hopper in the pic attached weighs 4.6 tons, and the 6 pads are quite easily capable of keeping the thing afloat.