If they've never been called before, simple distress should call them in, even better than a howler. After ya kill a few with the distress and miss a few, then break out the howler as your ace in the hole sound. Each time you call and miss, they will get harder to call next time, so don't show them all your tricks to quickly!
Sometimes they won't come. Some blame the moon, some blame the barometric pressure, some blame everything except thier calling. Sometimes, it's all of the above, plus some. If they came every time we blew on a call, what fun would that be!