What causes children to not respond to multiple vaccines and thus need 3-4 shots for the same vaccine?

Low immunity: Some children possess a weak immune system making them not respond to vaccines, i.e., they have difficulty building antibodies against the vaccine particles despite a scheduled regimen. They should be under close supervision of an immunologist either to boost their immune systems or advise them of precautionary measures to prevent them from contracting any disease.

Developing immunity: Babies are born with some passive immunity passed over the placenta from mom.That passive immunity is thought to limit the newborns ability to respond to most vaccines until they reach 6-8 weeks. They must be given again several times because the young immune system provides only transient protection. Once it matures around 2, less frequent boosters are needed. Some may need more later.