Pistons sometimes pull down a slime block and other times do not

When I place a piston under a slime block, and then two pistons next to it, like in the picture, and then extend it (and the redstone is correct) it will push the slime block up, then when I try to retract it sometimes it will pull down the slime block and other times it will not. It is not a directional glitch, and the redstone works on others, but sometimes, at random, it does not pull the slime block back.