How come the 'for' loop doesn't detect a certain string in the code I am current using?

I had been experimenting with the for loop, and sub method, when an idea occurred in my head, Is it possible for the for loop to detect a certain string within a string? And if so, will it replace the word?, and, sadly, the code did detect the string within the string, however, it only replaced the first letter of the certain string, and not the whole. I am currently confused on why that is, because, I thought I had coded it right, but, sadly, it did not end up like I wanted it to, here is the code I used in my experiment;

local msg = "I'm a Test! :D";
local chk = "";
local find = "Test";
local Replacement = "string";

for i = 1, msg:len() do
    if msg:lower():sub(i,i+find:len()-1) ~= find:lower() then
        chk = chk .. msg:sub(i,i)
    else
        chk = msg:sub(i,i+find:len()) .. Replacement
        print("FOUND MATCH")
    end
    wait(.1)
    print(chk)
end

print(chk)