Babies tend to nurse well as long as flow is rapid.  The younger baby
will tend to fall asleep at the breast when the flow is slowing, but
can still get some milk and the mother may get some more letdowns, so
the baby will then start to drink more.  The older baby starts to pull
off the breast when the flow is slow so will not get any more milk.
This behavioural change has all sorts of implications for the
breastfeeding.

So, what's up?  It could be that the baby just does not nurse long
enough.  Make sure the mother is not on the pill, which can reduce
milk supply.  Have her feed the baby using breast compression to
continue the milk flow.  Try blessed thistle and fenugreek.  Try
domperidone if you are not in the US.

I would stop the bottles of formula, which will only make the
breastfeeding worse.  If the baby really needs more calories, then
solids seem more appropriate to me at 4 months of age.  I would have
tried the above before adding the food.  What exactly was the big rush
to get the baby growing a little more?

Jack Newman, MD, FRCPC