Here are some other pictures.

And another

Why aren't you fertilizing?

Check the roots....are they rotting?

Josh

The roots aren't rotting. They seem nice and healthy, there's no bad smell from the soil or anything.
I am not fertilizing because I am afraid of over fertilizing as there might be fertilizer in the soil already.

Well, since you have several, I suggest you begin fertilizing one of them at least. Avocados are hungry plants, very vigorous, and so they can make use of nutrients.

That first one may already be too far gone.

Josh

Okay , I will try but I am fairly sure it is not the fertilizer.

I can see clearly that there is a nutrient impairment in the first image. Very obvious by the color and pattern of the leaves.

Most likely a soil that holds too much water compounding the issue.

Josh