To guarantee that the airline keeps your return you are required to check in and travel on the first legs and all the way to your final destination.
For example, if you booked London - New York - London, you cannot use the return as a single ticket from New York. You must fly London - New York first to be able to travel back on the ticket from New York to London.
The airline will automatically cancel all return flights if the passenger has not used the outbound flight and there is nothing that we as a travel agency can do about this. You will not receive any refund on the ticket either.