I am not a farrier by any means, but I really think that the way a horse's hooves are trimmed directly impact how they grow out.
I say this due to experience with my own horse. I have had him trimmed by 3 different farriers. With one of them, he was very sore afterward every time. With another one, his feet would crack/chip terribly, when they never did before. With the other guy, his feet were fine. If I trim his feet myself (and I am not a professional farrier by any means), his feet are fine.
Why he had soreness issues and cracking issues with the other two? I think it's due to their trimming technique.
And I suppose it is worthy to note that my horse is coming 14-years-old and he has been barefoot all his life.
It is good that you are adding supplements to her diet. I think maybe the next step would be trying another farrier.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.