Hello and thank you for your inquiry.
Let me start by saying you don't need to use the air intake on the powerheads, it is optional and most people don't run them especially on saltwater tanks. When used on a saltwater tank it can cause salt to spray all over the place causing a hugh mess.
Having said that, if you want to run the air intake it is very important to have the powerhead towards the surface of the tank. The venturi force used to pull air in is not very powerful and if the powerhead is too deep in the tank there isn't enough force being produced to pull air in against the water pressure. If you increase and decrease the depth of the powerhead in the aquarium you should see the effect this has on air production.
If there is anything else we can do for you please let us know.
Marine Depot Customer Service