Tails take for ever to grow back in properaly. I've been waiting 4 years for the cut portion of my horse's to grow in and it wasnt even cut that badly. Now it is is finaly grown back in and touching the ground. So you pretty much have a long road ahead of you. I would recommend not putting anything in your horse's tail for a while and seeing what that does. Sometimes their tails just grow better when left alone... Its worth and shot. The MTG obviously isnt working and you wont loose anything if you let it go natural for a couple of weeks. Personally I dont use MTG because one it smells and two it gave my horse terrible dandruff. Have you checked to see if someone isn't nibbling on her tail. Babies do that sometimes. A good friend of mind put MTG in her horse's tail and her filly ate it right up. That could be happening and you might want to look into it.