Pool / spa combo - having a problem with 1 jet, no air

Remove the jet, check for debris in the jet, run the spa without the jet, reinstall the jet, and see if it clears the problem.

There may be some debris in the jet, or the jet is not properly seated in the venturi.

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Show us pictures of the spa, jets, and nozzle.

Then your plumbing is good, and there is a problem with the way that nozzle interfaces with the air and water in the venturi.

Remove a nozzle that works and move it to the problem jet.

Compare the two nozzles.
 
What pumps do you have feeding the spa jets?

What speed is the pump running at?

Is the problem jet st one side of the spa?