I'd say there are two things going wrong here.
1) it's a bit unreasonable to generate 32 physics objects on top of the same location and not expect some problems
I change it only in the example. In the real project I generate the squares in randomly Y positions
2) Box2D web seems to definitely have trouble with this, locking the objects at odd locations.
If you switch to Box2D asm.js, as the engine, it actually works better.
(Also you need to get rid of the one object that is already in the layout, to control generating specific numbers.)
I changed the physics engine to Box2D asm.js and I think the problem is gone, but I tested your capx too and least squares is being generated (21 on average instead of 32). Why?
A last doubt: I tested the game on mobile and I saw that the movements of the squares is slow. It is because of physics behavior, right? Is there any way to optimize this? Thanks in advance.