SKSpriteNode will not appear when its physics body is dynamic - swift

Question

The SKSprite will only appear on the scene if .isDynamic = false otherwise it will not show up. Although it does not show up, it seems to be contacting with the other nodes in the scene.
I have tried playing around with the x and y values of the "lemming" node but this still results in it not appearing.
The Question:
What do I need to add or remove in order to make the "lemming" node be affected
by gravity, appear in the scene, and contact correctly with the "cliff" node?
In the GameScene.swift file:
//MARK: - Nodes
var background = SKSpriteNode()
var cliff = SKTexture()
var lemming = SKSpriteNode()
//MARK: -Catergories
let lemmingCatergory : UInt32 = 0x1 << 1
let cliffCatergory : UInt32 = 0x1 << 2
//MARK: - Start of game functions
override func didMove(to view: SKView) {
backgroundColor = UIColor.green
scaleMode = .aspectFit
size = UIScreen.main.bounds.size
physicsWorld.gravity = CGVector(dx: 0, dy: -9.81)
physicsWorld.contactDelegate = self
addBackground()
addCliff()
addLemming()
view.showsPhysics = true
}
//MARK: -Node customization
private func addBackground() {
background = SKSpriteNode(imageNamed: "background")
background.name = "Background"
background.position = CGPoint(x: frame.midX, y: frame.midY)
background.size = UIScreen.main.bounds.size
background.physicsBody = SKPhysicsBody(rectangleOf: background.frame.size)
background.physicsBody?.isDynamic = false
background.physicsBody?.affectedByGravity = false
addChild(background)
}
private func addCliff() {
var cliffNode : SKSpriteNode!
cliff = SKTexture(imageNamed: "cliff")
cliffNode = SKSpriteNode(texture: cliff)
cliffNode.name = "Cliff"
cliffNode.position = CGPoint(x: frame.midX, y: frame.midY)
cliffNode.size = UIScreen.main.bounds.size
cliffNode.physicsBody = SKPhysicsBody(texture: cliff, size: cliff.size())
cliffNode.physicsBody!.categoryBitMask = cliffCatergory
cliffNode.physicsBody!.collisionBitMask = lemmingCatergory
cliffNode.physicsBody!.contactTestBitMask = lemmingCatergory
cliffNode.physicsBody?.isDynamic = true
cliffNode.physicsBody?.affectedByGravity = false
cliffNode.physicsBody?.pinned = true
cliffNode.physicsBody?.allowsRotation = false
addChild(cliffNode)
}
private func addLemming() {
lemming = SKSpriteNode(imageNamed: "lemming")
lemming.size = CGSize(width: lemming.size.width / 15, height: lemming.size.height / 15)
lemming.position = CGPoint(x: 0.5, y: 0)
lemming.physicsBody = SKPhysicsBody(rectangleOf: lemming.frame.size)
lemming.physicsBody!.categoryBitMask = lemmingCatergory
lemming.physicsBody!.collisionBitMask = cliffCatergory
lemming.physicsBody!.contactTestBitMask = cliffCatergory
lemming.physicsBody?.isDynamic = true
lemming.physicsBody?.affectedByGravity = true
addChild(lemming)
}
func didBegin(_ contact: SKPhysicsContact) {
print("Body A: \(String(describing: contact.bodyA.node!.name))")
print("Body B: \(String(describing: contact.bodyB.node!.name))")
}