How can i prevent bosal from rubbing?

No matter what I ride my filly in - bosal, halter, sidepull - she always gets rub marks under her chin. Sometimes they get irritated and bleed a bit when I ride her multiple days in a row. I absolutely am not rough on her at all seeing as she's quite responsive off seat and leg cues. I'm assuming she has sensitive skin seeing as I haven't had this problem with any of my other horses. I share equipment with another girl who has a young horse and she doesn't have these problems, despite the fact she can be quite harsh on her horse. What can I do to prevent this? I always put a salve on her to help heal after I ride but I don't want her to look beat up in the show ring. My bosal piece is a dark brown. Would putting a layer brown/black vet wrap over the bosal help?