get some hoof cream and hoof oil by absorbene. My horse had the same problem and the medical hoof oil helped tremendously!
Any good oil based hoof paint before the go out and before you wash them after you ride.
Put shoofly wraps on. Thats the only problem is fly stomping. That suggestion came from the best podoatrist in ky
Our Farrier has us use Farrier's Formula hoof supplement. And I also used Venice Turpentine directly on the hoof, but it has to go on a dry hoof so first thing in the morning if your horse is in a stall at night is the best time to put it on. I recently had the same problem and applying it twice a day for a week really helped harden the hoof
I give my horse Horseshoers secret and his feet are rock hard.
Make sure your horse's hooves aren't too wet, keeping them somewhere dry may help. There are also farrier hoof feed supplements that can help a horse's hooves grow stronger.
Maybe talk to your farrier? He might have some suggestions for different shoes to use or a product that may help