4-5 months is still fairly new for a dragonette. Plenty of pods to your eye does not mean a sustainable population for a Mandarin.
What size is the tank? What steps have you taken to mitigate disease? What is the ammonia, and temperature? Is the light schedule vastly different from the store you bought it from? Whats your feeding regimen? Are the parameters that you say stable or just todays reading, if you think they are stable how often have you tested? Do you have a hard glass top? How long was the fish in the store prior to coming home with you? Was there anything unusual in transit, drop the bag, high or low temperature, long transit or acclimation time? Have you observed any abnormal behavior? Was there any white spots, black spots, red areas, hazy areas, open wounds, torn fins? Were fish on the same system from the store showing signs of illness?