I know it's happened to you and it's frustrated you. You give back a test, you discuss one of the more frequently missed questions, hoping for a teachable moment. But half the class is rooting through the rest of the test, sitting back with a vacant expression, or simply absent mentally. What to do?
One option, which I've discussed before, is to allow corrections for half credit. Then there's no need for you to take too much class time to go over the test -- it's the students' job to figure out what they missed, and to convince you they understand now. A related idea is to announce a "fundamentals quiz" over commonly missed concepts from the test. Either way, the students are forced to think about the test beyond just "what did I get?"
Of course, test corrections are time- and manpower-intensive. You have to give time in or out of class to get the corrections done, you have to grade them as thoroughly as you would a test. I only do corrections in my AP class -- I find the general class moves slowly enough that those who missed important points will pick them up soon.
So how do I go over a test in general physics? Well, keeping my comments brief and to the point helps. But the key little trick is to HOLD THE TESTS IN MY HAND while I go over them.
Here I'm playing with the students' minds. They desperately want their tests back, but only so they can see the grade. Once they see that grade, their mind is done for a while, and they don't want to think about physics. So I use the grade as a carrot. I dangle the papers with the grades on them right in front of the class. Not obviously or obnoxiously, of course, but they are never sure when I'm going to shut up and hand out the tests. And, they're nervous about what they did right or wrong.
So they listen. And ask questions. They want to hear what I say, so they can figure out whether they were right or wrong. The same discussion AFTER I give the tests back would be fruitless.
How do I know this technique works? Well, I don't for sure. But I do note that folks occasionally note to their friends whether they did or didn't make the mistakes that I discussed... so they must have paid some attention.