Hey Matador,
The American guys I studied abroad with in Sevilla a few years ago had the same trouble...they were so mad that all of us American girls were getting Spanish guys left and right but they kept striking out! Make sure you´re not dressing too "American"...I know that sounds shallow, but when I asked my Spanish girlfriends why they were not interested in American men, that was their number one reply, "their pants are too big". Go figure.
What neighborhood do you live in? I could give you some pointers on good bars to go to in Sevilla. We're going for Christmas this year and cannot wait!!!!!!!

Saludos,
Sarita