I had this issue with Piper and a few suggestions from trainers I asked.
Put her up on a chair and bring the food down below the chair so that if she wants it, she had to lay down. Because Piper was so small when trying to teach she food on the ground she could get from a sit position. But moving her to a chair, she had to lay down to get the food that was below the chair. (I hope that makes sense, its not coming translating well from my head to the fingers, to the keyboard
).
Another way they told me and this one I didn't use so I don't remember it as well, is to make a V shape with your leg and make sure pass under it and work on the down command. If you google you'll find the correct way to do it, but just an idea to get your wheels turning.
It took Piper FOREVER to learn the down command because not much worked.