Did you mean that you use an iron drench on your trees? If not, you might want to try that, as the soils in some sections of Florida are so alkaline that they might not be helped much by foliar sprays.

The University of the Florida suggests the following. "In high-pH, calcareous soils, chelated iron in a soil drench should be applied. For young trees, mix 0.5 to 2.0 ounces (14 to 57 g) of iron chelate in sufficient water to move the material into the soil around the base of the tree. For mature trees mix 0.5 to 4.0 ounces (14 to 113 g) of iron chelate per tree per application. Iron should be applied 2 to 4 times during the growing season."

Here is a link that might be useful: Lychee Growing in the Florida Home Landscape

Yes I meant iron drench and feragro spray. Thank you for responding.