Simply put, the "piston trapdoor" setup used to make Slime block Elevators doesn't work in various situations.

As shown in the first setup, when the block the lever is on is at (146,60,-1531) and the whole thing is facing east (lever to pistons), the top piston doesn't retract the slime block.

However, as shown in the second setup, when the block the lever is on is at (39,56,-1541) and the whole thing is facing south (again, lever to pistons), the top piston does retract the slime block.

In the first setup, a 1/2 tick Redstone pulse from the Observer will not cause the slime block to be retracted but it will in the second setup.

From what I have observed, placing the lever on top of a block directly next to the pistons will not retract the slime block in any circumstances. However, placing it on the underside of a block next to the top piston does work.

The seed for this world is 335243319869194455 and the super flat preset used is the Redstone Ready preset.