Failure to thrive is an actual disorder. Most often it is a runt. There are many possible causes and sometimes no cause is found. Underdevelopped offspring can have digestive tract issues, neurological problems, lack of sucking reflex, poor waste removal so toxins build up, various organ problems, heart failure.... Who knows what it is. A vet might be able to find something but whether it can be fixed is another matter. Mothers often stop feeding runts or the runts just give up fighting with their siblings for milk. Especially in a large litter like that. Hand feeding might work if it's only problem is being weaker than the others but it's possible the puppy is just not a viable offspring and will die from internal problems. Something you have to face when breeding.
