How to stack sprite elements without being dropped thru object

I’m trying to stack some sprites, let’s say that they’re boxes, but when I stack more than 5, the lowest ones fall thru the supporting object.

The idea is to stack as many boxes as wanted, but it seems that I’m missing something to tell to the physics engine.

You can see the working code here: https://codepen.io/dlaguna/pen/xMyzVW

Changing the numElems variable to 5 shows how the scene may work.

Any ideas? Thank you!