visa decision dispatched but still not received

Hi can anyone help me?

I received an email on 11/12/17 that my visa decision has been dispatched on 7/12/17 but until today I still have not received my passport and decision letter.
I tried contacting UKVI but they said they cannot help me as the decision has already been made and it depends on the postal service. However my VAC said that UKVI has not even dispatched my passport back from the decision making centre! What should I do?