Originally Posted by
javabytes
Whenever I've had issues like this, it's because of KLM enforcing married segment logic. Whether that's actually supposed to be happening when booking with DL miles, I couldn't tell you... but I've had plenty of agents who found availability when searching point-to-point but not when searching for the entire journey, and when they tried to piece it together by segment KLM denied the sell.
This would mirror PMNW, where at least for upgrades with miles, I was told that married segment logic did apply for intraEU KLM flights. In particular, to get certain intraEU segments upgraded, I needed to avoid nonstop MSP-AMS flights at times.