Originally Posted by
MSPeconomist
There have been reports of even the KLM inventory showing on EF not being bookable with DL miles, although this can be the result of DL agents not knowing how to do it. Personally, at the end of last summer, I saw KLM award flights on delta.dumb, but was unable to ticket them online and the DM line couldn't get me these award flights either. It looked consistent enough not to be a problem with dynamic inventory availability.
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.