Some possibilities:
*She's too engorged for bub to get a grip on - try expressing partly and then attaching
*Bub expects instant gratification - try a small bottle feed and then the boob, or try a supply line (need LC help with this)
*Let down is too fast or too strong - hand express for 30 seconds or so into a towel and then try and attach
*she tastes wrong - nipple shield
*bub has a tongue tie - lactation consultant should pick this
*some other issue that makes sucking uncomfy for bub - e.g. ear infection or thrush - check temp, see GP.
I found the lactation clinic at Jessie Mac to be awesome; also the Ringwood MCHN's are pretty good.

hugs
Kate