iOS Swift didBeginContact not being called

I have been struggling for the past two days to get two SKSpriteNodes to register a collision and evoke didBegin#contact.

I've set their bit masks 'categoryBitMask', 'contactTestBitMask' and 'collisionTestBitMask' for both objects.

I've also set the 'dynamic' property for both to 'true'

initPhysics() seems to set up the physicsWorld okay.

All I'm expecting is that didBegin#Contact is called, but it is not

//Set up Physicsbody bit masks
let playerCarBitMask: UInt32 = 0x1 << 1
let slowCarBitMask: UInt32 = 0x1 << 2



//initPhysics
func initPhysics() {
    println("(((((((((((((( Initiating Physicsbody ))))))))))))))")
    self.physicsWorld.contactDelegate = self
    self.physicsWorld.gravity = CGVector.zeroVector

    println("self.physicsWorld.contactDelegate = \(self.physicsWorld.contactDelegate)")

}

//setupPlayer
func setupPlayer() {

    car = SKSpriteNode(imageNamed: "redCarUp")
    car.setScale(2.0)
    car.position = CGPoint(x: 800, y: 400)
    car.zPosition = 100
    car.name = "car"

    gameNode.addChild(car)

    let carBody = SKPhysicsBody(
        rectangleOfSize: car.frame.size, center: car.position)

    carBody.dynamic = true
    carBody.categoryBitMask = playerCarBitMask
    carBody.contactTestBitMask = slowCarBitMask
    carBody.mass = 5
    carBody.collisionBitMask = slowCarBitMask
    car.physicsBody = carBody

    println("carBody = \(carBody)")
    println("carBody.dynamic = \(carBody.dynamic)")
    println("carBody.mass = \(carBody.mass)")
    println("carBody.categoryBitMask = \(carBody.categoryBitMask)")
    println("carBody.contactTestBitMask = \(carBody.contactTestBitMask)")
    println("carBody.collisionBitMask = \(carBody.contactTestBitMask)")



    slowCar = SKSpriteNode(imageNamed: "blueCarUp")
    slowCar.setScale(2.0)
    let slowCarScenePos = CGPoint(
        x: 680,
        y: 2048)


    slowCar.position = gameNode.convertPoint(slowCarScenePos, fromNode: self)
    println("slowCar.position = \(slowCar.position) ****")
    slowCar.zPosition = 80
    slowCar.name = "slowCar"



    let slowCarBody = SKPhysicsBody(
        rectangleOfSize: slowCar.frame.size, center: slowCar.position)


    println("slowCar = \(slowCar) ****")

    slowCarBody.dynamic = true
    slowCarBody.categoryBitMask = slowCarBitMask
    slowCarBody.contactTestBitMask = playerCarBitMask
    slowCarBody.mass = 5
    slowCarBody.collisionBitMask = playerCarBitMask
    slowCar.physicsBody = slowCarBody
    gameNode.addChild(slowCar)



}

func didBeginContact(contact: SKPhysicsContact!) {
    println("*******************PhysicsContact********************")
}

It turned out to be a simple problem. In my original code I was setting parameters for the SKPhysicsBody detection frame like so:

let carBody = SKPhysicsBody( rectangleOfSize: car.frame.size, center: car.position)

Similarly I was doing the same for the second node that I was testing physics collisions for.

Simply removing the 'centre:' parameters like so:

let carBody = SKPhysicsBody(rectangleOfSize: car.frame.size)

for the two sprite nodes solved the problem and the nodes now crash into each other and push themselves aside as expected.