The function won't return anything?

This is my first time learn about recursive function, and I wrote a test code:

local function func(a)
    a = a + 1

    if a == 100 then
        return a
    elseif a > 100 then
        return 100
    else 
        func(a)
    end
end

print(func(0)) --return nothing

The weird thing is it return nothing. Nothing in the output, not nil or anything, just nothing.

Here is another script that do the same thing, but it work:

local function func(a)
    for i = 1, 100 do
        a = a + 1
        
        if a == 100 and not a > 100 then
            return a 
        else
            return 100
        end
    end
end

print(func(0)) --return 100

Did I do anything wrong or I'm just retarded.

Thank for reading :D