I have a 9 year old spa (Brett Aqualine EM-190, electronic) with dual pumps. There are no air bubbles when I open the air control knob for the high speed pump jets. Jets appear to still have a very strong water flow. Do I need to replace the air control or could there be another cause for this?

likely to be a problem at one or more of the jets
are all jet inserts in place?
any internal jet seals/o-rings missing or broken?
if these are a problem - air will not venturi for all jets on the same air control