Try to pull up your reservation on klm.com and see whether you can select your seat there. I had an AF flight booked as part of a DL reservation and the AF website didn't allow seat assignements (all the seats showed as not available), but I could get a seat on the KLM website. Right now I have a new reservation in economy, booked on DL, with two KLM flights (BER-AMS and AMS-BER), which are marketed as DL-flights. For the first time ever, I am not able to select seats for these flights on DL. When I go to the AF website, no seats are available for selection, while KLM.com asks me to pay for them (which also didn't happen before). Interestingly, when I try to make a new booking on DL.com for the same route and dates, I can select a seat on the BER-AMS flight during the booking process, but not on the way back. When I was making my reservation on DL, I didn't bother to select seats, and I didn't know this wouldn't be possible later.