Oh golly have I been there! without mastitis, but persistent breast refusal.

One of my favourite tricks is to squirt a bit into baby's open mouth, and then try again.
With DD1 we had to limit her night feeds to improve the day feeds. It's a long story, and took us ages to work it out, but it was affecting her weight gain and general happiness.
With DD2 I have had to break up her feeds and offer short, frequent snack feeds. She just seems to get full and pull off really quickly. Doesn't sound like it's your problem, but just throwing ideas out there to see if anything sets off a light bulb!

Could it be a sore neck issue? Would osteo help?