A float plane's take-off is no different than a land based plane.
It must be able to rotate in order for the wing to lift the plane of the water/ground at a given speed,
At a slower speed more angle is needed. At higher speed less angle.

First assuming your step is in the proper range

If the afterbody (part of the float from the step back) of the float won't allow the plane to rotate easily, it will take much more speed in order for the wing to make lift at the flat angle of attack

Think of it this way
Imagine a Taildrager with a vey tall tail wheel that held the tail up so high the plane is almost at flying additude (you may have seen this at you land feild)
Well the plane may never be able to rotate enough in order to come off the ground.

A float plane will act the same way if the tail is forced to stay high by the afterbody of the float

Thats why those land speed record cars never leave the ground. Put their mains close to cg and they would launch.

It's all about the angles, Well the cg/step location is very important aswell.

Shim the front gear in small incriments until you find the perfect angle and you'll be Golden!

P.S. I wouldn't ever use Gee bee floats (rounded edges issue), but I have seen guys use them with ok results on lite aircraft. Good Luck