Why does free chlorine of water getting reduced at the receiver end even having good concentration at discharge point?

as water with good concentration of free chlorine from the discharge point traverse thru the line leading to the receiver end it scour all the bacteria in the line, more if the piping is GI pipes the more free chlorine will be reduced due to oxidation of Iron from the pipe.
Hope that answers the question.
So as a remedy you can first check the free chlorine from the discharge point and the receiver end up to what degree it was reduced, from there you can decide how much you can increase the chlorine injection from the discharge point so as to maintain a0.2-0.3 ppm free chlorine at the receiver end.