Janet,  prematures can have a quite weak suckle for many reasons.   I
see a good suckle coming from three things, compression, some suction
as the jaw lowers and creates a larger intraoral space and an adequate
seal from the tounge and lips

Prematures have proportionately smaller bodies/heads and hense mouths,
tongues and lower jaws - making it difficult for them to compress the
areola far enough back to really be effective, especially if compounded
by mom having any "anotomical challenges" (broad nipple, sinuses far
back from nipple, even long nipple, flat nipple or heaven help us,
inverted nipple, in any of the many permutations).

Prematures also have less or no brown fat storage (laid on during the
last trimester), meaning they lack energy reserves AND more to the
point of their physical suckle abilities, the brown fat stores in the
cheeks are missing, making the intra-oral space of a premature
relatively larger in spite of their small over all size.  So...they
need to work harder to creat suction, which does play some small part
in milk transfer.

I don't know if their little lips have trouble creating a seal, the
third component to a good suckle, but perhaps... Does anyone know when
the pars villousa (those little lip ridges that help the baby "grip"
the breast) form, relative to gestational age.  Now that I think about
it, I BET they form late in gestation, as other "ridges" do.  Fetal
scrotal and labial tissue is smooth at younger ages, as is the fetal
areola.  I'll bet the lips are too, but I'm just guessing here.

Paula Meier said she is using nipple shields to help prematures for
their lack of ability to really pull the nipple out and hold it.  The
use of a breast pump on the opposite side, simultaneously, would
certainly help with the lower stimulation that the nipple shields
cause.

Prematures are also sleepier creatures in general than full terms,
making some additional breastfeeding problems.

Carla (just north of Washington, DC, USA)