When you call on a student who has raised her hand to answer your question, you might be signalling to other students that they don't need to think when you ask questions in class.
Why? When we ask a question in class, we want all our students to think about an answer. But if we routinely call on students who volunteer information, then other students quickly learn that they can let their more eager peers do the thinking for them. They just have to wait it out. And just 'cold-calling' kids out of the blue may catch a few kids who weren't thinking, but it won't necessarily make your whole class think.
But we can up the stakes for the whole class by deliberately calling on students randomly. And there's a Notebook tool that does the grunt work for us: it's called the Random Word Chooser.