aww. yeah one thing Tucker never had a problem with is down. He would do this belly flop thing where he'd sit on his back legs and slap himself to the floor.
Are you allowed to use treats? (I ask because once I went to a class where you couldn't use treats). If so, put the treat on the floor *while they are sitting* and move it away from them slowly. While you're doing that, put your finger just below their neck a put a little pressure on it. While doing that just keep saying "down" and when they do it, throw a mini party!
and if you can't use treats, just skip the treat part and hopefully the rest will work for you!
Good luck!