Are you sure you are getting a full letdown? If it is a plugged duct hook up to a pump and use wet heat and massage to work it out. The first month after birth I came in to the ER with an infection. They though it was because I was getting mastitis (it wasn't) but the lactation consultant helped me out with the clog. I hooked up to a pump and used hot, wet rags and massage to work it out. I pumped for like 45 minutes before I got the few lumps worked out. I was super sore from the massage afterward but it worked and it didn't clog again.
There is a supplement you can take to help with clogged ducts, but I don't remember which one it is? Lecithin? Anyone?