Looking at the plumbing, I'm wondering if its because the air line that feeds the 2 jets that don't have air coming out is lower than the air line around the rest of the spa. Maybe water is filling up the air line on those 2 jets and won't blow out.

Is there any way you could hook a shop vac (in reverse where it is blowing) and feed it into the air line to blow any water out to see if that is what's causing the problem?

Just a thought....

JD