Heat and humidity may affect the outcome of your bath fizzy.
Start with the minimal amount of water recommended. Add additional mists of water as needed until the fizzy formula clumps together when squeezed. Too much water will cause your formula to activate - less water is better. Add water as needed until the right consistency is achieved.
You will need to press the powder down firmly into the mold. There should be enough powder to make 3 each batch. They will fall apart if the formula does not clump together when mixing with your hand and if you do not press them together enough when adding to the mold.