I would retrain the down completely.

At first do not expect anything. Get her into a sit, have a treat in your hand, and let her sniff it. Lower it infront of her body until you get her in the down position. TONS of treat at praise. Do NOT add a command until she is regularly and consistantly downing without the treat and just a hand signal(hand going towards the ground). Then you can add the down command. Or, to prevent any issues with using this word, you can always choose another word.