Upvote:3
The rule is that if you fail to travel any segment of your trip, the rest of the trip auto-cancels. That applies on most scheduled transport including trains. This is to combat "hidden city ticketing": often, for marketing or subsidy reasons, Bumblebutt-SFO-JFK is cheaper than SFO-JFK, or a round trip is cheaper than a one-way.
If you didn't know about this rule and just booked some random one-way, that's on you. Because it's your job to know that rule. That is why it is in disclosures they give you.
However (I'm taking your word here) you rescheduled. That means you contacted the carrier, told them of your change of plans, and had them change your ticket for the earlier departure. In that case, yes, they screwed up and it's on them.
Upvote:15
The use of your ticket is covered by the "Contract of Carriage" of the airline.
Air Serbia, like most airlines, including a condition in it's contract of carriage as follows :
3.3 COUPON SEQUENCE AND USE
The Ticket you have purchased is valid only for the transportation as shown on the Ticket, from the place of departure via any Agreed Stopping Places to the final destination. The fare you have paid is based upon Air Serbia Tariff and is for the transportation as shown on the Ticket. It forms an essential part of Air Serbia contract with you. The Ticket will not be honoured and will lose its validity if all the Coupons are not used in the sequence provided in the Ticket.
Given that you failed to use the first "Coupon" (ie, the outbound flight), any subsequent flights on the same ticket are also invalid.
Whether you are due a refund will depend on the exact ticket, but in most cases a refund would only be possible if you notified the airline that you were not planning to make the initial flight BEFORE the time that flight was due to depart. For most tickets, once that flight departs without you, then your entire ticket would have no value.
The only people who can really tell you if a refund is due is the airline themselves (or possibly the travel agent if it was booked via one), but as I said above, it's very likely that nothing is due.