"So how do you distinguish between a flare and treatment failure?"
I have always felt that a flare is a temporary treatment failure due to stress, temperature fluctuations or a new food. If it happened too often or in more than joint at a time, my rheumatologist asks me to take a a short course of prednisone to calm the flared up situation down. That usually helps to restore the treatment coverage. If it had happened too frequently, then I suppose I would have concluded that the treatment had completely failed and I needed new or different medications.