How to drag 3D models smoothly

Hi guys,

I have a problem when dragging 3D models.

I'm using UIPanGestureRecognizer. Like code below.

But when I drag the object. I can drag it up but I can't drag it down.

I don't understand why?

@objc func didMove(_ recognizer: UIPanGestureRecognizer) {
        let location = recognizer.location(in: sceneView)
        let arHitTestResult = sceneView.hitTest(location, types: [.existingPlaneUsingGeometry,.estimatedVerticalPlane,.estimatedHorizontalPlane,.existingPlane,.existingPlaneUsingExtent,.featurePoint])
        if !arHitTestResult.isEmpty {
            print("You're dragging")
            guard let selectedNode=(hit?.node) as SCNNode? else { return }
            let transform = arHitTestResult.first?.worldTransform
            let newPosition = float3((transform?.columns.3.x)!, ((transform?.columns.3.y)!), (transform?.columns.3.z)!)
            selectedNode.simdPosition = newPosition
            sceneView.scene.rootNode.addChildNode(selectedNode)
        }
    }