Trouble with Tilemap collisions in P2

[saumanahaii]

I'm having a bit of trouble with tilemap collisions.  I'm using P2 and a json tiled tilemap.  Strangely, it seems to work with small, simple tilemaps but as soon as I boost the complexity it seems to stop working.  I've got a simple car being driven by physics that turns one way when you touch and the other when you don't.  Help?  Here's the create code:

  create: function(){
    game.physics.startSystem(Phaser.Physics.P2JS);

    //!THIS STUFF WORKS WITH TEST MAP
    this.map = this.add.tilemap('city');
    this.map.addTilesetImage('roguecity');
    this.decorationLayer = this.map.createLayer('decoration');
    this.collisionLayer = this.map.createLayer('collision');
    this.collisionLayer.resizeWorld();

    this.map.setCollisionBetween(0, 12);
    this.physics.p2.convertTilemap(this.map, this.collisionLayer);

    this.stage.backgroundColor = "#d3d3ff";
    this.cursors = game.input.keyboard.createCursorKeys();
    game.input.keyboard.addKeyCapture([ Phaser.Keyboard.SPACEBAR ]);

    //map stuff

    //create the layer
    //this.backgroundLayer = this.map.createLayer("background");
    //car stuff

    this.car = game.add.sprite(60,35,"car");
    game.physics.p2.enable(this.car);
    this.car.anchor.set(0.1,0.5);
    this.car.body.allowRotation = true;

  },

Thanks!