The question was posed to me a couple of days ago, about teaching a dachshund the "down" command.
Sounds like you have the command down,(no pun intended) but you want to lengthen her response time? To do that, what I do, especially with pups that have short attention spans, once I get them into the down position, I'll put down a hand full of treats. This keeps them in the down position a little bit longer than giving a tiny treat.
I did a video of Texas doing going through some of the behaviors I've taught him. And each time I have him "down" I put down three or four treats, which, as you'll see keep him the position longer.
This will also roll over into the "wait" or "stay" command. Putting down a handful of treats, walking away while the pup eats them, and coming back right as they are finished.
I'll let an expert demo that one!
I Hope this helps!