This is a simple rule that takes a bit of practice to get used to – every time the camera moves, you need to write a new slug line. When I was first starting to write scripts, it didn’t occur to me to pay attention to exactly where each scene was taking place. I would write a slug line each time there was a new master location, but then would just describe in action when the characters were moving around that location, such as, "Charlie walks outside and gets into his car." I didn’t realize that I needed a new slug line each time that the action moved around the same general place.
The most common ways that I see this mistake now are when characters move in and out of buildings, among different rooms in the same building, or in and out of cars. Each one of those transitions needs its own new slug line.
Alternatively, a great way to denote when you're changing locations within a single master location is to write just the name of the room that the characters are moving into. For example:
INT. JOHN’S HOUSE – DAY
John and Diane stumble inside, pausing to tear each other’s shirts off and kiss in the doorway.
Diane drags John down the hall, kicking off her heels as she goes.
Diane shoves John onto the bed and launches herself at him.
Ronald watches John and Diane from behind the bathroom door. He flosses as he enjoys the show.
Did that last example have to be about sex? No, it did not. Did there need to be flossing? Yes, because oral hygiene is very important. Besides, gratuitous sex can add interest to a seemingly lackluster topic, which was hopefully all you needed to help you remember to write a new slug line or one of those mini-slugs every time the cameraman has to haul his equipment to a new location. And I'm serious about the flossing.