I'm on the level where you push a rock down zigzagging wooden planks an…

I'm on the level where you push a rock down zigzagging wooden planks and then it seems it should fall into a pit at the bottom. That would fill that pit up so when you go get the stone you need to sit on the door opening button, you'd be able to roll it over that pit and get it where it needs to be. But, my first rock always disappears once it drops off the planks. Then I'm stuck once I go get my other rock. Please help!