Not a bug. Seat upgrade offers are not guaranteed just because there are open seats. Have frequently seen cases where they are not being offered on certain flights. Your only option in these cases is to do a re-fare (which will also upgrade all flights on a single fare component -- for a roundtrip booking, typically the outbound flights will be on one fare component and the return flights will be on another). You can attempt yourself online with change flights option, but if it's an agency issued ticket it will likely require DL taking control of ticket.