For the last step of the flush, did you water with a weak solution of a complete fertilizer with micros? That's an important step. In your case, I'd use a bloom fert for the flush, since I see no evidence that the plant is N deficient at all. I'd also drill some extra holes in that pot to dry it out faster. This is especially important in winter when it's cold. I'm seeing a lot of cold/wet combo related issues come through here since the weather has turned wintry.