A decrease in appetite is a cause for concern. It is often difficult to determine how much your bird is actually eating and how much is on the bottom of the cage. A bird who completely stops eating is in serious trouble and needs vet care immediately. Clear discharge from the nares can be from simple sinus irritation caused by dust, pollen, etc. Continued discharge, or yellow or green colored discharge can be a sign of a more serious problem which requires vet care.
All birds sneeze on occasion.
Many birds will insert one of their talons into their nares to force a sneeze. Continued sneezing is not normal and a vet should be called. Inactivity can be caused from lack of sleep or from a more serious underlying problem. If your bird doesn't perk up after a few hours of rest then you should call your vet. Dull feathers, abnormal feather growth, or loss of feathers not associated with molting can be a sign of a vitamin or mineral deficiency caused from diet, lack of natural sunlight, or a more serious problem affecting the vital organs, or from Beak and Feather Syndrome.