Ajet Refund - Paid Seat Reservation Failed

I reserved seats on 15 September for my round-trip flight from Düsseldorf to Istanbul and back. I flew outbound with Pegasus Airlines (22 September) and return with Ajet (26 September). I reserved seats because my wife prefers the three of us (with my 4-year-old child) to sit next to each other. With Pegasus Airlines the seats were fine, but with Ajet I was given different seats and we did not sit next to each other, even though I paid for that. I want a refund for the seat reservation on the return flight.