Shapes

So I'm fiddling with some blocks today while playing with my daughter, and I discovered that if I use 4 of the same shape, I can make a bigger version of that shape.

I had successfully done this with 5 different shapes, when I got to the yellow hectagon.

It didn't work.

The triangle, square, and diamonds worked. Even the red one worked (what's it called?). But the yellow shape, wouldn't do the trick.

I thought I had discovered some secret "rule of four". After all, I was taught to scientifically prove something, you have to do it 3 times and get the same results. I had FIVE positive tests, and ONE negative.

I'm just glad I did that sixth test before making a fool out of myself ;-)

But why won't it work?
Is there some mathamatical formula or rule?