Similarly, I have 2 yellow flowering Mandevilles less than one metre apart: one is thriving the other is struggling, why? I dunno, I treat them both exactly the same: feed them, give them both the same dose of trace elements. It's a mystery! When I lived in Sydney (Northern Beaches area) my cosmos thrived. Are both your varieties subject to the same conditions? ie. sun, shade, food, water, soil, etc?