I had the same problem .
My farrier told me to (after treating the thrush of which I used ACV ) get some memory foam , bout 2cm thick and cut to the shape of my horses hoof and tape it on with Duct tape. Leave it on for a few hours every day .... Depending on how your horse reacts is weather you leave them stabled , turned out or take them for a walk.
The memory foam forces frog stimulation & frog stimulation creates growth .
You do need a new piece of foam each time but its pretty cheap .
My horse has all her frog padding back now , It worked fantastically !