Fizzing performance is closely linked to freshness and proper storage. Bath bombs react when citric acid and bicarbonate come into contact with moisture. Even humidity in the air can slowly activate them.
Bathrooms are one of the worst places to store bath bombs because they tend to be humid from showers. If a bath bomb absorbs moisture while still in its packaging, it can partially activate and lose some of its fizz before you even use it.
For best performance, store your bath bomb in a cool, dry and dark environment. Keeping it in an airtight container before placing it in a cupboard is ideal. Proper storage helps preserve the full fizzing effect.