not an expert but their terms say:

"2. Fully Refundable Tickets

If your ticket was purchased at a fare that is fully refundable, Delta will issue a refund of any unused refundable portion of your ticket at your request. You must surrender any unused portion of the ticket at the time of the refund request. No refund will be issued on any ticket unless Delta receives a request for the refund and any unused coupons are surrendered to Delta within one year of the original issue date of the ticket. The amount of the refund will be calculated as follows:

1. If no portion of the ticket has been used, Delta will refund the total fare and all taxes, fees or charges paid.

2. If a portion of the ticket has been used, Delta will refund an amount equal to (a) the total fare and all taxes, fees or charges paid, minus (b) the fare and taxes, fees or charges for the used portion of the ticket."

Of course.. take screenshots of the app that isn't working too...