Raymond -
There's several things to consider with this issue. Since pushing dirt is an exercise loaded with variables, let's try to eliminate a few of them by trying something different. What happens when you try to push into a pile of settled topsoil or gravel? Can you fill the bucket before it stops?
As for spinning the tires, remember that spinning the tires is what you're trying to avoid. The EarthForce machines aren't like tractors. They're full-time 4WD and have a very different weight distribution than typical tractors. Depending on the soil conditions, it's quite possible that it will have more traction than the engine/transmission is capable of exceeding. At least that what I've found with my EF-5. Despite this occasionally being the case, it's always accomplishing a lot more work, or exerting a lot more force than my Kubota L4310 was capable of producing. What I'm saying is that not being able to spin the wheels is decieving as an indicator.
On the other hand, if you can't fill the bucket with gravel, given decent soil conditions, it may be that it's not producing the power it should. The EarthForce machines use a hydrostatic transmission by Rexroth with a control system called "automotive control". What it basically means is that the output of the pump is determined by engine speed and back pressure from the motor, while output from the motor is controlled by its internal pressure. The net effect is that maximum torque is only produced at maximum engine speed. So if, for some reason, the engine isn't producing rated rpm, you won't get maximum torque from the transmission. There's also the possibility that the relief valves in the motor and/or pump aren't set properly, but that will require dealer servicing to fix.
I hope this helps. Let me know what you find out.