Training small dogs can be tricky. One command that people tend to find especially difficult with smaller dogs is the down command. With traditional food luring, or leash pressure techniques, the dog may get confused because they are already so close to the ground, so it is difficult for them to grasp what you are trying to communicate.
The following video shows a very simple trick that I frequently use to teach small dogs the down position. It is very easy, can work on any dog, and is conflict free.