Marilyn, if the trim is right, then I would suspect a metabolics issue, perhaps related to diet.  I have a flat-footed horse.  He can’t handle a lot of green grass or rich carbohydrates.  When I keep his diet very simple with just free-choice Bermuda hay; a handful of whole oats/BOSS; and a supplement, I see his concavity improve.  There is just nothing in his diet to undermine hoof wall attachment. 

Here is an excellent website that provides some really good insight on the connection between grass and hoof problems. 

www.safergrass.org

I also have set up a “Paddock Paradise” that keeps my horses moving 24x7.  It has been awesome.

Linda