
As I understand it there are some dogs that the vaccination will never take.
To be honest re-vaccination is most likely to take when the immune system is mature, so usually over 6 months of age.
I think this is the logic of boosting at this point in the USA.
The puppy protocols are based on the assumption that maternal antibody levels will have fallen enough for the vaccination to take, this process can be very variable.
It also appears that some breeds seem to have this happen more often, dobes and Rotties being two I have heard of needing a booster after 6 months as the puppy vaccs haven't taken.