Why does anhedonia occur even when you are not on medication?

I know anhedonia is normally caused by a deficiency in dopamine in the brain? But what part of the brain? Is it dopamine disregulation? Are you dopamine cells dying like as is the case in Parkinson’s disease? And why are medications used to fix anhedonia only temporary- ad in they fizzle out in their effectiveness.

I feel like invega killed my dopamine re up system😕