According to Wikipedia:
Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in).
Typically, I put the dish inside the oven in its center. I suspect most people do the same:
Now, because the plate is in the center, it will remain more or less stationary as the oven's dish rotates.
In my case, this causes only part of the food to be heated, only certain areas. Other parts remain cold.
What I've done to solve this is not place the dish in the center, but on the edge:
This fixes the problem and my food is more uniformly heated.
I suspect that this is because of the long wavelength which, combined with a stationary food plate, make certain areas of the plate unreachable.
This makes me wonder: Is there a specific reason why oven manufacturers don't switch to a shorter wavelength ? (let's say 4cm) Are there any essential physical properties of the water or fat molecules that would prevent shortening the wavelength ?