Why does my Wonder Pot now bake unevenly? I've been using a Wonder Pot for a year and a half, mainly making cake and the occasional bread.

Recently my cakes have started baking unevenly, with one half of the pot completing the baking before the other half. I usually end up turning the pot 180 degrees when it is a little more than half done, in order to evenly bake. This was not an issue when I was first using it. 

Anyone experienced with Wonder Pots know why this might start happening, and what can be done to fix it?

I have recently changed stovetops, and it is possible I'm not getting the flame height right, or the positioning (although it is pretty centered).

It was already used when I got it, so perhaps the metal disc is worn out and no longer distributing the heat evenly. (Does that even happen)? 

You can see pictures of the top and bottom of my disc here. I'm not sure if corrosion or rust is the right way to describe it, but you can see that there is some kind of something on the disc, and it appears a little uneven. It doesn't seem all that different from the picture on wikipedia, though.
Your flame may look even, but check the heat diffuser component of your wonderpot in your photo - you see the sootmark? That's a clear indication[PDF document from Association of Home Appliance Manufacturers] that the flame isn't burning cleanly... soot is made of carbon byproducts that should have been consumed by a clean burning flame, which consists of both fuel (cooking gas) and air. The burner's job is to combine both to make a clean flame. That much soot means it's not burning efficiently, which leads to uneven heat.

It can also lead to the production of carbon monoxide, which is not a good thing.

You will need to clean and calibrate the burner so it produces a clean, pure blue-white flame that burns clean on the bottom of your pots, or replace it if that doesn't help. This should only be done by someone knowledgeable in gas appliance repair.