Crickets and blank faces when I ask questions in class - help!

[TíaJulia]

I've been teaching for 12 years but have run into a problem at the school where I currently teach. I teach high school Spanish and have run into this problem only with certain groups of students...

...I have a good rapport with the students, no problems with classroom management, grades are overall looking decent so far so I know the kids understand the material. I just can't get over this one barrier. With some groups I have plenty of active participation in class. With other groups I get a sea of blank faces and total silence! (But when they take tests or quizzes or write papers it's clear to me that they know the answers, they just don't want to speak up.) During class discussions I call on people if I don't get volunteers, but it's still like pulling teeth and they REEEALLY don't want to answer (even though 9 times out of 10 their answer is correct).

These kids are friendly and sociable with me in the hallway so I don't think they hate me or anything. I just can't figure out how to get them to speak up in class. I have tried every trick I can think of. I think it's at least partly fear of ridicule by classmates (even though I've made it clear time and again that I won't tolerate students picking on each other). I've really tried to make it as painless as I can, and try to encourage students to work with each other and to be understanding since they're all in the same boat.

I get that at this age the idea of looking dumb in front of your peers is terrifying. I try to keep it from being too traumatic but I need class participation of some sort! I even try to incorporate games and make class fun, but they still won't budge! Anybody out there had the same problem and found a good solution? How do I get these kids to speak up?