I just wanted to agree with Daveedka's post and that one other possibility for yellowish leaves could possibly be a magnesium deficiency. But until you are actively dosing iron and still seeing a problem I would not consider this a problem yet. The reason I mention this is that Magnesium deficiency can affect the uptake of Iron in plants, thus appearing EXACTLY THE SAME, because they are still not able to process the iron, even if it is available to them in the water.
As suggested by most, try dosing either more Flourish or Flourish Iron in addition, then if things still don't seem to change for the better, then try adding some Magnesium. If this is the problem, it will supply what is needed by the plant to process the available iron.
I have recently run into a similar issue with a customer's tank, except they use well water, so I'm first adding more Flourish , then I will add Magnesium as needed if this doesn't perk it up in a week or two. Most tap water supplies this, but well water is questionable as to any nutrients it supplies. You really have to watch the plants and see what they tell you.