my four whell drive doesn't work

I got a kia sportage ex 2000, 120.000 miles, 2.0 liter, 4 cylinders and my four whell drive doesn't work, my mechanic says the axle moves and it has a automatic locks in the both sides of the tires and no one is broke but when I put the four whell drive only rear side moves.
So I already tride riding on the snow, going back and forth and the front tires won't move, unfortunately I don't have the owner manual on me and i appreciate it you help me aup with it, I want to know what could be the problem.