I can't prove it, but I feel that gluconate chelators aren't so effective in water with higher KH levels. I was using Seachem's Flourish and Flourish Iron in a tank with a KH of 15 for a while and then gave TMG a try. I did notice quite a difference.

There was much less of a difference between Flourish and TMG in water with a KH of 5, though subjectively I still prefer TMG.