if you already use the word 'down' for something else then try using the word 'flat'.
entice them into a flat position by holding the treat by their nose and slowly moving it down towards the floor - as soon as they hit the right position use your key word and give them lots of praise.
we had to change a few of our key words very early on to avoid confusion. we find these work best
sit
down (saying down again gets their heads down too

)
off (for off the furniture etc)
speak (barkies for a treat)
finger on lip for a quiet speak
come (recall without a whistle)
2 pips on a whistle for a recall
wee wee's - for the toilet training
a firm 'aaahhh' for interupting bad behaviour
paw
'and the other one' (for the other paw)
'up' for jumping in the bath etc.
fetch
Short and simple
we're still working on our 'roll over' and 'go and put the kettle on' but they get there in the end
You might already know, but training is much easier if you use an extra tasty treat that they dont normally get - small cubes of cheese, warm chicken, chopped up hot dog sausages all work great - but save them just for training and dont spend too long each time trying.
Also dont use the keyword too often if they arent responding as they just learn to associate the keyword with ignoring you