Corona Physics problem

local physics = require("physics")
physics.setDrawMode("hybrid")
physics.start(true)
physics.setGravity(0,9.8)

display.setStatusBar( display.HiddenStatusBar )

local bkg = display.newImage( "bkg_cor.png" )

local grass = display.newImage("grass.png")
grass.x = 160; grass.y = 430

local grass2 = display.newImage("grass2.png") -- non-physical decorative overlay
grass2.x = 160; grass2.y = 440

physics.addBody( grass, "static", { friction=0.5, bounce=0.3 } )


local crates = {}
local stopit = false --make it true to stop instead of pause, you wont see problem
local displace = 0

--local dropCrates = timer.performWithDelay( 500, newCrate, 100 )
function startagain()
    physics.start(true)
    if stopit then
        local grass = display.newImage("grass.png")
        grass.x = 160; grass.y = 430
        physics.addBody( grass, "static", { friction=0.5, bounce=0.3 } )
    end
    if displace == 0 then
        displace = 10    --change this to > 30(since crate dimension is 30), won;t see the problem
    else
        displace = 0
    end

    SetCretes()
end



function CreatePhysicsProblem()
   for i=1,#crates do
       crates[i]:removeSelf()
       crates[i] = nil
   end
   crates = nil
   crates= {}
   if stopit then
    physics.stop()
   else
    physics.pause()
   end
   timer.performWithDelay( 1000, startagain )

end

function SetCretes()
    local j = display.newImageRect("crate.png",30,30);
    crates[#crates+1] = j
    j.x = 60+displace
    j.y = 405--335
    physics.addBody( j, { density=3, friction=.500, bounce=0} )

    local j = display.newImageRect("crate.png",30,30);
    crates[#crates+1] = j
    j.x = 60+displace
    j.y = 368
    physics.addBody( j, { density=3, friction=.500, bounce=0} )

    local j = display.newImageRect("crate.png",30,30);
    crates[#crates+1] = j
    j.x = 60+displace
    j.y = 330--402
    physics.addBody( j, { density=3, friction=.500, bounce=0} )

    timer.performWithDelay( 2000, CreatePhysicsProblem)
end

SetCretes()