One of my mares freaks out when you try to clean between the teats. She will squeal a little and move away if you just try to touch them. It is the only place on her body that she is touchy about. Any ideas?
If it's just between them, has she been explained by a vet in that area for possible ouchies down there?
When I first got Lacey and was unsure about how she was going to react to teat touching, I would use a long lunge whip so I could stand at her head and rub the end of the whip around her side and under to her teats.
If she had been touchy, I would have just desensitized her to the whip touching her teats with me at her head, then I would have slowly worked my way down to her flank area (still using the whip as an arm extension), then once she was 100% with that, introduce my hand down there.