Have the dogs in question actually been taught the command heel yet?

They don't know that they have to stay within the range of a loose leash by your side if you don't teach them. I mean, my dogs and any other dogs I walk certainly aren't expected to stay at a perfect heel by my side, but once I taught them the command and started using it, I never had a problem with heeling since. What I do after teaching the command is when the dog starts to pull, I'll stop and let the dog hit the end of the leash then tell it to heel. Pretty soon the dog learns that if it wants to walk, then it needs to stay within the range of the loose leash.