Example demonstrating the issue mentioned on the corona forum: http://forums.coronalabs.com/topic/47975-admob-only-shows-first-banner/

ad1.lua

local composer = require( "composer" )
local scene = composer.newScene()
local ads = require "ads"
local widget = require "widget"

local nextScene = function()
	composer.gotoScene("ad2")
end

function scene:create( event )
	local sceneGroup = self.view

	-- create a widget button (which will loads level1.lua on release)
	playBtn = widget.newButton{
		label="switch scene",
		labelColor = { default={255}, over={128} },
		width=154, height=40,
		onRelease = nextScene	-- event listener function
	}
	playBtn.x = display.contentWidth*0.5
	playBtn.y = display.contentHeight - 125
	sceneGroup:insert( playBtn )
end

function scene:show( event )
	local sceneGroup = self.view
	local phase = event.phase
	
	if phase == "will" then
		ads.show( "banner", {x = 0, y=display.contentHeight - 50, appId="__INSERT_DUMMY_APP_ID__"} )
	end	
end

function scene:hide( event )
	local sceneGroup = self.view
	local phase = event.phase
	
	if event.phase == "will" then
		ads.hide();
	end	
end

function scene:destroy( event )
	local sceneGroup = self.view

	if playBtn then
		playBtn:removeSelf()	-- widgets must be manually removed
		playBtn = nil
	end
end

---------------------------------------------------------------------------------

-- Listener setup
scene:addEventListener( "create", scene )
scene:addEventListener( "show", scene )
scene:addEventListener( "hide", scene )
scene:addEventListener( "destroy", scene )

-----------------------------------------------------------------------------------------

return scene

ad2.lua

local composer = require( "composer" )
local scene = composer.newScene()
local ads = require "ads"
local widget = require "widget"

local nextScene = function()
	composer.gotoScene("ad1")
end

function scene:create( event )
	local sceneGroup = self.view

	-- create a widget button (which will loads level1.lua on release)
	playBtn = widget.newButton{
		label="switch scene",
		labelColor = { default={255}, over={128} },
		width=154, height=40,
		onRelease = nextScene	-- event listener function
	}
	playBtn.x = display.contentWidth*0.5
	playBtn.y = display.contentHeight - 125
	sceneGroup:insert( playBtn )
end

function scene:show( event )
	local sceneGroup = self.view
	local phase = event.phase
	
	if phase == "will" then
		ads.show( "banner", {x = 0, y=display.contentHeight - 50, appId="__INSERT_DUMMY_APP_ID__"} )
	end	
end

function scene:hide( event )
	local sceneGroup = self.view
	local phase = event.phase
	
	if event.phase == "will" then
		ads.hide();
	end	
end

function scene:destroy( event )
	local sceneGroup = self.view

	if playBtn then
		playBtn:removeSelf()	-- widgets must be manually removed
		playBtn = nil
	end
end

---------------------------------------------------------------------------------

-- Listener setup
scene:addEventListener( "create", scene )
scene:addEventListener( "show", scene )
scene:addEventListener( "hide", scene )
scene:addEventListener( "destroy", scene )

-----------------------------------------------------------------------------------------

return scene

main.lua

-----------------------------------------------------------------------------------------
--
-- main.lua
--
-----------------------------------------------------------------------------------------

-- hide the status bar
display.setStatusBar( display.HiddenStatusBar )

-- include the Corona "composer" module
local composer = require "composer"

local appID = "__INSERT_DUMMY_APP_ID__"
local ads 	= require "ads"

local function adListener( event )
	-- event table includes:
	-- 		event.provider
	--		event.isError (e.g. true/false )

	local msg = event.response

	-- just a quick debug message to check what response we got from the library
	native.showAlert("Message received from the ads library: ", msg)

end

ads.init( "admob", appID, adListener )	

-- load menu screen
composer.gotoScene( "ad1" )