I have the return portion of a ticket from LHR-IND which I need to change the date on. It's an economy ticket (S). I now need to fly the same route on July 23 or 24, 2017. I have a very narrow window and little flexibility on the date.

The problem is that the system says no standard economy tickets for LHR-IND are available, at all. None for April, May, June, or July 2017. Only the more expensive flexible fares. Meanwhile, though, super-saver economy award seats are available. However, I don't want to use award miles, as I have this half-ticket that I've already paid for.

What gives here? What am I not understanding about why there are no economy seats? I need to make the change soon, as the flight I was originally booked on is coming up.