Why MathJax isn't working on Blogger

There are countless pages saying MathJax works under Blogger. But I couldn't get it to work, despite following the instructions carefully. My problem: I was hitting preview. I followed these instructions and then hit Publish, and everything worked great.

Everything breaks eventually. So, below I have the Laplace Equation in Cartesian coordinates so readers will know whether my method still works.

$$\frac{\partial^2 U}{\partial x^2} + \frac{\partial^2 U}{\partial y^2} + \frac{\partial^2 U}{\partial z^2}= 0$$