Are your tires loaded with liquid ballast? Do you have ag's or bar tires? Do you drag the BB in 4x4 mode?? What kind of terrain/material are you working with? All that makes a difference, but as Roy suggested, maybe you are taking "bites" that are too big at one time? One can probably stall out and make just about any CUT spin the tires with a well made heavy duty BB that is full of material. So maybe you need to adjust your expectations and stay within the limits of your tractor and BB?