On a wholly battery fed system the horn is generally more efficient and suffers fewer problems. Lots of older lightweight motorcycles and scooters and probably some of the more modern types either do not have a battery or do not have an effective charging system for the typically small battery with sufficient surplus capacity to provide current for accessories should the battery become discharged.

It is not unusual as these sort of bikes pass the first flush of youth to find the horn does not work very well particularly when the lights are switched on.

If there is a battery it should be fully charged and tested. The wiring connections from the generator, the connections to the battery, to the horn, to the horn push and the horn push contacts should all be thoroughly cleaned and in particular all the ground connections.

If the horn is in good condition and the generator is working efficiently the above maintenance should put things right. Many horns are adjustable and minor adjustments are likely to be needed throughout it's service life.