I fly AMS to DEL and back often. It seems at least 2/3 of the time I have a seat assigned in advance and then at check in KLM randomly moves me. Today it was from my nice 15E to a middle seat in 42. Nice! When I call Delta or KLM they both tell me its the other airlines fault and there is nothing they can do. What am I doing wrong? I always make sure my Diamond number is on the reservation. Then after I book I always call / Tweet BOTH Delta and KLM to confirm. Today KLM tells me "It is not possible at present to reserve a seat on a flight operated by KLM and bearing a Delta Airlines flight number. We are working with Delta on a solution". ...?

Try booking via KLM.

+1. Have a trip to JNB next weekend booked on KL stock and have been able to select EC seats for free with my DL Skymiles number on the res. We'll see if it sticks, but I'm more confident with a KL reservation than a DL one. OP, did you get the KL itin number with your DL reservation and check on klm.com to confirm your seat was selected?

Flying on KLM, always book direct with KLM.

KLM doesn't hate you, they hate Delta. It's not personal, it's part of the seamlessness of the SkyTeam "alliance."

How many years has it been since booking a KL flight through NWA.COM automatically yielded the KL PNR?

Someday DL will be profitable enough to fix its crap customer-facing IT.

How does that work when most of the itinerary is on DL metal?

For example, something I flew last year:

RDU-BOS-AMS-HEL

With AMS-HEL on KLM, the rest of DL. I couldn't get preferred seats on AMS-HEL, but given it's a relatively short flight I didn't feel like arguing too much, plus I had an aisle already. It was a 006 ticket.

Maybe I should've booked the whole thing as a KLM ticket?

Correct, by the entire ticket on KLM. Delta recognizes and treats you better on KLM stock than KLM does on Delta stock. It is a nightmare and you can do a search on many KLM threads about getting econ comfort seats for elites, and having your seats moved.

How does the upgrade on the DL domestic segments work out, when flying DL metal on KLM ticket stock?

The ticket stock isn't what matters except for mileage and mqm earnings. What matters is who is marketing the fare. Ie are you flying a KLM or DL fare. Both can be ticketed on either stock. A KLM fare will have all KLM flight numbers and a DL fare will have all DL flight numbers. In fact if you purchase a ticket from KLM by web anywhere or by phone in US/Canada with a N. American address the system will ticket on DL stock.

Upgrades for DL flights for DL SM Medallions should process the same regardless of who's flight number you're booked on.

The problem will come in using GUCs/mileage upgrades. In this case they can't waitlist you since you not on DL123 which is their flight and in their system. Rather you're on KLM123 which is a KLM flight that happens to be operated by DL123.

Finally for seat assignment problems in C+ and KLMs version, this split between being booked on one flight that's really another flight is what causes the issues. From an IT standpoint it's a nightmare and amazing it works as well as it does. From a customer standpoint, most will never understand the difference between KLM123 operated by DL123, and DL123 by itself.

I have never found this to be the case. I book on KLM.com frequently and it always tickets on 074 stock, not 006 DL stock even though I live in North America. But I agree with the rest of what you said, I put my DL SM # in on KLM.com and the itinerary usually shows up on my DL profile quickly with ability to pick seats etc.

A nice friendly agent at AMS told me what she believes is the real problem here. She said I was given my C+ seat when I booked but when check in opens for the local AMS folks if they want to BUY a C+ seat, KLM will sell them one and bump me out. I was shocked to hear this and didn't believe it until I thought back to the 3-4 times this has happened to me and it makes total sense.

The other upsetting thing here is that first KLM told me it was simply not possible. Then they changed their tune and said C+ was only paid and that I had not paid for it, then they came back (after I reminded them Diamonds get them complimentary) and said the plane was full and they had no seat. I was like, which story are you going to use here?

Free tip: Dutch people tend to think that the rules are meant for everyone else but not for them.
Another nice one of them is publish that there is no charge for SkyMiles Platinum or Diamonds for those Economy Comfort seats then block those first EC rows off for Delta.

So wait, can I not use GUCs for DL-operated flights on KLM ticket stock?

So I have EC seats on 2 out of 3 KLM flights and have been checking regularly for an open EC on the third flight. Checked today only to discover that KLM has locked down my seats and won't let me change even though it's more than 72 hours before my first flight. I understand the 48 hour airport control thing, but 72 hours+?? I'm now with OP, KLM must hate me.