When I meet a baby who can suck competently on anything but a breast (and the
breast contains plenty of easily flowing milk) then I suspect (a) the breast
(flat areola that needs to be compressed into a nice "breast sandwich" or
short inelastic nipples that need help - perhaps a shield) or (b) baby's oral
anatomy: tongue-tie, overly receding chin, short tongue or just a real small
mouth.  Does the baby stick its tongue out beyond the labial border??

I have used nipple shields many times (only those of a major manufacturer)
with great success but always watch a complete feed before letting mom go
away with one.  About every 20th time everything looks great but then you
notice the baby is not milking the breast well enough through the shield.
 Something to do with placement of the lactiferous sinuses?  Moms have no
trouble weaning off them eventually but it can take weeks if anatomy truly
problematic.

Hope this helps.

Joanna Koch IBCLC in Los Altos, CA