Common reasons for failure of immunity to develop against Parvo in puppies are:

1. Maternal antibodies from the milk still being present at the time of vaccination. This is especially true for pups 5 weeks or younger, and is the reason why booster shots are given. If maternal antibodies are present the attenuated virus in the vaccine is inactivated, and the puppy's immune system will never react to it.

2. Mishandling of the vaccine during shipment, and/or non-proper storage, causing a less potent vaccination.

A puppy probably isn't completely safe against Parvo until the vaccination cycle is complete.

However, even a weak vaccination or partial immunization should help the puppy to recover faster. But the most important thing you did was to react promptly and get the puppy treated. You have at least a 80% chance of a complete recovery if not better.