Strider did very well indeed on the Down Boy. In fact, every time I called him, he would come to me and prompty lay down and wait for his treat. It had nothing to do with the word Down but rather the treat he got when he did it!! He is very smart, isn't he?
And then he seemed to forget about "Sit"!
The dog trainer was ahead of me on this one. The following week , he told us to tell him "Down" , then "Sit" (and he had to get up and sit) and randomly give treats. That helped. We are also telling him to sit at odd times when he doesn't suspect treats are in order, then give him one.