Pistons not retracting Slime Blocks

The explanation lies with the update order. The top sticky piston tries to retract before the bottom right-facing sticky piston, so the slime block connects to the bottom upward-facing sticky piston, and the movement is blocked by the extended arm of the bottom right-facing sticky piston.

This behaviour is unique to slime blocks, so normal blocks work the same. In order to get this device to work with slime blocks, simply remove the bottom right-facing sticky piston. 🙂

@edit Dang you Kabo! I just finished writing and you explained it already 😉

I'm not sure if that can be called intended but here is how it happens.
It's just an update order thing.

At first the top piston tries to retract at first but can't pull the block because the arm of the lower piston is still in the way (As the slime block sticks to the upwards facing piston). Therefor it only retracts the arm.
After that the lower piston gets updated and retracts the block in front in as normal.

I your case I would just get rid of the lower piston as the upper one will retract the slime block and the upwards facing piston anyways.
Other than that you could try to make sure that the lower piston gets an update first (trigger it a tick earlier or something like that).

"Working by Design"

Is there a chance this will get fixes in the future? And yes, I understand it means changing the redstone update order.