KL is big on marrying segments like this. I had the same scenario using CO miles for KL SFO-AMS-BCN. SFO-AMS was OK, AMS-BCN was OK, but SFO-AMS-BCN was not OK. I was trying to go using CO miles, and nothing would work, so we booked just SFO-AMS (stopover for a few days) AMS-BCN, BCN-AMS-SFO. That worked (the return had no problems).

What dates are you looking for? With all the partners, on such a heavily travelled route, hopefully there's something out there that will work for you.