Interesting, Mausie. I admit I'm stumped. I've had ears that stand up, go hound, ears too far apart, but I've never seen one fall back. You're doing all the right things. How is the ear leather? Is it very thin? Perhaps if it's thin and weak, that's the reason it's falling. You might consider a supplement to strengthen the leather if that's the case. I've heard calcium does that but I'm reluctant to overdo that with a puppy because of the danger of compromising bone structure so I'd use it very judiciously.
Two methods I've heard of for correcting hound ears that may help you: use a Breathe Right strip to prop the ear up, or cut a piece of plastic from the zip end of a small plastic bag and use that. Good luck and let us know how it goes! It's always helpful to hear new methods of solving these things.