I second the idea of ultrasound. I had reoccuring blocked ducts caused by a bra and I needed two courses of ultrasound with 24 hours and it finally resolved the issue. A lot of physics don't know about it, but my GP told mine the intensity level required etc (or you could google it).

I think at this stage it is the best solution for you. Also make sure bub is fully draining that breast. Maybe express after a feed from that breast just to be sure.