As seen in the screenshot, one set of pistons pulled a slime block, whereas the other did not for some reason. This redstone elevator operates by using two sticky pistons to push a sticky piston and a slime block. The pushed piston pushes the slime block right after, launching the player upwards. In the space of one tick, the pushed piston pulls the slime block back down. Finally, the first 2 sticky pistons are supposed to pull both the piston and the slime block back now that the system has retracted. I even used obsidian around the pistons to be safe, but all too often the two pistons only pull the moving sticky piston and not the slime block. Considering that there were only two items being pulled back at a time, and I'm playing offline, my guess is that the game doesn't like a sticky piston and a slime block being pushed beside each other since the slime block pulls things with it  but in this case that shouldn't be a problem since both are being pulled in the same direction. As shown, this happens about half the time.