See Jan DeCoopman's book Breastfeeding Management for Health Professionals.
She has an excellent chapter on slow weight gain and interventions r/t
excessive initial loss, early onset slow weight gain, impending failure to
thrive, persistent slow weight gain, late onset slow weight gain, high
calories demands.
She has explored this and thought it out carefully.  Slow weight gain at
different ages and stages indicate different causes.  Pat in SNJ