It's possible that there's better availability using Worldperks miles than Skymiles, but in my experience whenever I've found an available KL flight on the NW site, the DL partner desk has been able to book it for me. Also depends on what route you're trying to find seats on - LAX-AMS seems particularly difficult whereas JFK-AMS or ORD-AMS is relatively easier. I'm booked DL LAX-ATL and KL ATL-AMS. I typically book 3-5 months in advance.