Generally, only saver (the lowest price) awards are available to partners, but Flying Blue makes more saver award space available to members of their own program. If you're searching on the KLM site, saver-level space that you're seeing may only be available if booked through Flying Blue. Searching on Delta is indeed a better option, since if you can see award space on a partner outside the program, presumably it will also be available to Skypass as well.