pistons cant pull slime

I was working on making a new elevator for a build. and it required a piston to push a piston with a slime block on top out, so the piston could push the slime up but when i did so. the 2 back pistons pushed the slime and piston out. the piston pushed the slime block up and pulled it back down. but when the 2 pistons tried to pull them back in, the piston came back, but the slime stayed in the same place. please fix.