Ok, so I know this may sound odd, but maybe she just isn't his type.
Since they spend so much time together, could he simply not see her in a "romantic" way.
I've worked for many horse breeding farms and trust me when I say horses absolutely have preferences. Had one stallion that did not like mares with spots, like Paints or Appaloosas. One that really loved the palominos. So I'm thinking it could be possible for dogs to have a similar preference.