Fiona,
Other things I like to check on when a mom has recurrent plugs:
        baby's suck:  I have seen babies with higher muscle tone and tighter
sucks being associated with plugs near the areola/nipple.  relaxing the muscle
tone by gently bouncing or rocking the baby before feeding can be helpful, as
can feeding the baby while he lays on his back and mom kneels over him.
        mom's bra/purse/anything that would put pressure on the breast in that
area and keep that duct from draining.
        baby's frenulum: sometimes if it is just a little tight it can
interfere with stripping the milk from the breast.
        if all else fails, I encourage the mom to pump with a hospital quality
breastpump before or after nursing on the plugged breast.  I have found
lecithin helpful for most moms with recurrent plugging who have tried it.
Catherine Watson Genna, IBCLC