I used to have that problem too until I switched up a wider traction tire on the rear. I run 2.35 Bontrager on the front which gives good stability and cornering and a 2.5 Delium on the rear which is seriously knobby. There are slopes of slick mud I can climb now with barely any tire slip that I would spin out prior to changing. Tire pressure is a definite factor also, especially in sand and scree/