When something is not lessened, it is undiminished. You might be hoping that the pain in your ankle will go away but find that even after a day of rest the pain is undiminished. Probably time to see a doctor.
To diminish means to lessen. If you were stranded on a desert island you might worry about diminishing supplies of food. If you had endless supplies of rice but nothing else, it would be pretty surprising if, after eating nothing but rice for a week, your love for the grain was undiminished.