A few years ago, I bought tubers of Kelvin floodlight. They limped along for a few years, and this spring I divided the cluster, giving my daughter half. Her plant has produced large, golden-with-streaks of red extremely double flowers. Mine produced one of those but is mostly producing SINGLE, unattractive blooms. What gives?
This may be a case of different soil types. The soil in which she planted hers may either have more nutrients available, or a more stable pH.
Add some dolomitic lime to the area around yours, and I bet that solves the issue. My guess is that the pH is out of range.