ANA does generally interline to partners and are normally quite good at it. What probably happened is that the agent noticed that the next ticket had no free baggage allowance and thus did not want to check the luggage through, as they did not have the option to charge the luggage fee on behalf of United.

Mostly if luggage is checked through, the receiving airline will not notice the missing payment and it will just move through the system. But checking in across tickets is not a waiver for luggage fees on the second ticket so it does happen that agents does not want/are not allowed to check through to a ticket without free allowance.