Flies wont stay away?

I go out the barn everyday. its been about 95 degrees out for about two weeks now and the horses stay sweaty needless to say. but i've noticed that my horse gets COVERED in flies everyday. more so than the other horses and they're all out in the pasture just the same. i curry and brush his whole body down to the hooves everyday and spray him all the way down with fly spray but its like the flies dont even notice it. literally 5, 10 minutes after spraying him there are flies covering him again and i dont know what to do to make them leave. sometimes i use repel x and sometimes i use bug block they are both good fly sprays. anyway its making me upset. what do i do?