If there are no air bubbles in the chamber and the nozzle is well below the surface then I would go back to an inspection of the tubing - even the smallest pinhole could be drawing air into the line. Thoroughly check any connection in the return tube where the tube is above the water line. This may even mean pulling the pump, return line, and nozzle out of the tank (if possible) and reconstruct it in another container (e.g. 5 gallon bucket may work) with the water line at the same level on the tubing and also with the tubing completely submerged. Unless some amount of air was trapped during the initial installation and hasn't worked its way out yet, the air has to be coming from somewhere.
#reefsquad