It is not uncommon for dogs to refuse to walk sometimes when they are outside. They might just "put the brakes on", or they might sit or lie down.
If your dog is a really young puppy, you might want to just take a break and let them take the world in. You do not have to worry about your dog becoming lazy. This is not a sign of a personality trait that leads to a lazy older dog. Often it revolves around a puppy being ever overstimulated or tired. A puppy's energy level moves up and down very quickly. He might just need to take a break. Use this time to socialize your dog to the world passing by. If time is a factor, just pick up your puppy and go home.
It is nice, however, to know how to get your dog moving when you need to. I have had many clients call over the years because their dog lies down in the middle of the street. This can be a harrowing experience, that is for sure.
Here are some tips to help motivate yoru dog to keep moving. It is important to practice these techniques periodically on every walk. Not just when the undesired behavior is happening.