Have this problem with some of my horses and not others. I use a turnout fly sheet, a fly mask with a nose covering and feed smart bug off. The smartpak brand. In extreme neck cases, I use Endure fly spray but not everyday. Try Endure on the neck 2 times a week, If you need more, you can use it everyday but that’s usually not necessary. Try adding one day a week until you reach the amount you need.