I had a bit of trouble teaching Luigi the down command as well. I made a post just a couple weeks ago asking how I can get him to stop lunging at my hand for the treat, and just stay down
A few days after I made that post, he finally got the command!
What I did at first was hold the treat and place it down on the floor so that he would actually get down on the floor. Then eventually, I would keep the treat in my left hand, and kinda gesture with my right hand (lowering my right hand a little) and this seemed to work. EVerytime he'd try to lunge at my hand, I'd just say "NO!" and hold him back and wait until he was patiently lying down. That's what worked for me.
Maybe someone else will have more advice for you
But I am sure Chase will get it soon...I thought Luigi would never get it (even though I had only taught him for 2-3 days haha..but he did!) Just keep at it! Good luck!