If your baby is feeding on breastmilk and yet doesn't seem to be gaining weight, it could be that your milk supply isn't enough. It can be due to stress or fatigue. You can speak to your baby's paediatrician or a lactation consultant for some tips to help improve your milk supply. If your baby is formula-fed, problems with digestion or incorrect measurement of the formula can be a possible reason for slow weight gain. Your paediatrician or lactation expert will be the best person to let you know for sure.