If she understands the word, use a toy or tug instead...play tug, out her and then tease her up. Give her the down command(only ONCE) and mark it when she does it. Then start tugging again. If you say the command over and over it loses strength. If she is down in a straight posture, push the toy into her and say down/back which should make her bounce back in the down position. That may help her do it faster from a sit or a stand.
Ramp her up good to get the faster positions.
Edit, I see she's in teething mode, so tug should be gentle, but it still can be done gently if pup likes that game.
^^^Another good suggestion!
I alternate between toys and food with my pup.