Jose, I suspect that you vet is trying to find out if there may be
lymphoma or a pancreatic infection as well.  This might not be straight
insulinoma situation.
 
If the surgery and the Prednisone are not working trying Proglycem makes
sense, as can tryng the two in combination.
 
You may also want to discuss actually giving sugars with your vet.  Along
with the treatments based upon hard data, there have also been fashions
in treating low blood sugar levels over the years, largely because many
things tried have been extrapolated from animals which are taxonomically
far from ferrets (like humans) or based upon hypotheses.  For a while
over a decade ago the mode was to give sugars.  (We had one girl, Fritter,
during this time who had pancreatic lymphoma and she had sugar given
throughout the day as well as high Prednisone doses every 8 hours and
Proglycem every 12 hours, and the combo gave her around 9 months.)
Later the most discussed approach became to not give sugars to avoid
an over-response by the pancreas which would then possibly produce yet
more insulin.  (According to ferret vet pathologist, Bruce Williams,
insulinomae themselves are independent of blood sugar levels.)  Your vet
may want to test the ferret and see how independent of blood sugar levels
this individual's insulin levels are.  If they are markedly independent
then giving sugars as part of the routine may be the way to go for this
particular individual.  Discuss it with the treating vet.
[Posted in FML issue 3724]