Similar thing happened to me, I am AF FB Plat and was travelling to the US on a Delta ticket (AF metal). since it was a Delat ticket I had to handle my seat assignment throught Delta. Beside a wide availability of prefered seat at the front they always declined to assigned one to me. This is in violation of the Skyteam Elite Plus benefit for preferred seating. I tried calling their hotline in the US for the way back and ended up talking to a guy who had no clue at all about skyteam and partner benefit (Indian Hotline???).
In addition to this I was booked on a B class ticket therefore almost full fare coach

One more time DL disapointed me and whenever I can I ll avoid them