Pistons don't push/pull Slime blocks

Slime blocks are pulling the ground along with the slime block, and that explains why you can't push the slime blocks. Try them in the air instead on the ground. That's a feature of the slime block, to pull blocks in the same direction the slime block is being pushed. But it can't push blocks over a limit of blocks possible. Go to this link for more information:
http://minecraft.gamepedia.com/Slime_Block