This sounds to me like a premature baby.  A baby who is even a few weeks
early is likely to sleep a lot and be difficult to keep awake during
feedings.  Since this mother was on Brethine (terbutline) and you don't tell
us the gestational age, my best guess would be that you are dealing with
prematurity.  Many women have uncertain dates, and menstrual and ultrasound
dates often don't match, so even some supposedly term babies are actually
premature.  If this is the case, the baby will probably "wake up" at around
37 weeks post-conception age, at least my 34-weeker did.

Carol's comment about the possibility of magnesium sulfate use is also
important to investigate.  Magnesium not only affects the baby, but I believe
I have heard that it may also delay lactogenesis.  My experience seems to
bare this out.

With regard to research on terbutaline's impact, it would be important to
exclude anyone who had been on magenesium.  Bonnie