There are a few things that could cause this.

• The venturi and air inlet may need cleaning.
• Many foods or additives will effect foam production. Please click here for more information.
• If the aquarium is new or has low stocking levels foam production may be limited. As the tank matures or livestock is added this will change.
• The salt level in the aquarium may be low. Check the salinity and confirm your refractometer / hydrometer is reading correctly.