"water chemistry is perfect" means nothing. Give us specific numbers for pH, Total Alkalinity (TA), Free Chlorine (FC), Combined Chlorine (CC) and Cyanuric Acid (CYA) -- Calcium Hardness (CH) would be nice to know, but not critical for this problem.
What kind of disinfectant are you using? How much do you add and how often? Does your spa have an ozonator?