I have an older tub with 4 jets and 2 air intakes.

The intakes have screws tops which I open to allow air

Problem is only 2 of the jets will pull air in to mix with the water

It has nothing to do with the pump as the side that works does so with either low or high speed

It's not the jets, because I have swapped them and it makes no difference

I can put my finger on the jets that don't blow air and the water will shoot out of the intake, which makes me think there is no blockage

I can also run a small tube down into the intake and I can't find any blockage

Right now, I'm in the process of replacing the wood around the outside, so I have access to the plumbing

Any ideas on how to get the air going again ?