Is PRP necessary to maintain transplanted hair? What are the many reasons transplanted hair did not last pass 24 months?

These questions are for transplanted hair (not native hairs0. I know many who had good results within 12-24 months of transplanted hair (lower hairline, more density in temple) but suddenly the transplanted hair fall/stopped growing. Will PRP help these transplanted hair grow again? What are the reasons that the transplanted hair grew nicely but suddenly stopped and further thinning while the native hair 'seems' to maintain its density? Will second transplant help by covering the same area again.