E.A.
Any constriction can cause the ducts to block. Sports bras and underwires are the biggest culprits. What are you doing to unblock it?
My advice:
Take off the bra and go braless as much as you can.
Point the baby's chin toward the blockage when you nurse.
Lay the baby on the floor and crouch over him/her with your breast hanging down to nurse the baby, to allow gravity to help pull the blockage out.
Also, warm compresses.
Drink more fluids.
Good luck.