Process Monitoring in Elixir

Hi elixirians,
Here I am trying to monitor the process. Based on the official documentation I just tried with the follwing lines of code.
However the monitoring callback is not triggerd. Please help me to figure out this.

defmodule HelloServer do
  use GenServer

  ## Client API

  def start do 
    GenServer.start_link(__MODULE__,1, []) 
  end

  def add pid,value do
    GenServer.call(pid,{:add,value})
  end
  def get pid do
    GenServer.call(pid,:get)
  end

  def reset pid do 
    GenServer.cast(pid, :reset)
  end
  def monitor pid do 
    GenServer.cast(pid, {:monitor, pid})
  end
  def stop pid do 
    Agent.stop(pid)

  end

  ## Server API

  def init(initial_value) do   # initiating the state with the value 1 passed 
    {:ok,initial_value}
  end

  # add the value to the state and returns :ok
  def handle_call({:add,value},_from,state) do
    {:reply, :ok,state + value}
  end

  # returns the state to the caller

  def handle_call(:get,_from,state) do
    {:reply,state,state}
  end
  # just reset the state to value 1

  def handle_cast(:reset,state) do
    IO.puts "value has been reset "
    {:noreply,1}
  end

  # monitor the given process id
  def handle_cast({:monitor, pid},state) do
    IO.puts "monitoring the process"
    Process.monitor pid
    {:noreply,1}
  end

  # This executes periodically
  def handle_info(:work,state) do
    IO.puts " This message prints every after 3 seconds"
    schedule_work()
    {:noreply,state}
  end

  # this is triggered when the agent stopped the process

  def handle_info({:DOWN, ref, :process, _pid, _reason},state) do
    IO.puts "Agent stopped the pid"
    {:noreply, state}
  end

  #catch-all clause for the handle_info for handling unkown messages

  def handle_info(_msg, state) do
    IO.puts "unknown message"
    {:noreply, state}
  end

  defp schedule_work() do
    Process.send_after(self(), :work, 2 * 1000) # In 2 seconds
  end

end

This is How I am using the file in iex

iex> {:ok, pid} = HelloServer.start
iex> HelloServer.monitor pid
iex> HelloServer.stop pid             
# after calling this it should trigger the handle_info :DOWN ...
# but unfortunately not able to trigger that..
# am I missing something here ?

Thanks in advance.