The humidifier pro utilizes a hydrophilic sponge, which is denser than a regular sponge. Because of this increased density, when the sponge is dry, it can take a bit longer for the sponge to absorb water. When the sponge is completely saturated, it will become soft and malleable.
To help the sponge absorb water quicker, you can work the water in with your fingers. You should feel the sponge becoming soft as it absorbs water. Otherwise, leave the sponge submerged until it is completely soft. Be sure to squeeze out any excess moisture from the sponge after it is fully saturated, as the sponge should never be dripping when it is placed in the humidifier housing.