Here's one for the experts to explain. Why can't the slider in my demo pass by the motor? There's clearly room and no blocks infront to stop it. Yet the slider is stuck. It can go backwards and with enough tries it does pass the motor, but then can't go back again. Spoiler: My demo
This thread answers your question. Physical dimensions of Mekorama blocks Motor is actually a cube shape so when it rotates one of its edges goes outside of its own square grid, thus blocking the slider movement, but with enough force, the slider can still brush past it. The reason is because you had a stone block in front. Try placing the stone block in front further and drag the slider back and forth, it'll have enough force/momentum to go past the motor.