VNDetectTrajectoriesRequest not returning any observations

Code Block 
private lazy var pathRequest: VNDetectTrajectoriesRequest = { return VNDetectTrajectoriesRequest(frameAnalysisSpacing: .zero, trajectoryLength: 5, completionHandler: trajectoryHandler)}()

Code Block let pathHandler = VNImageRequestHandler(cmSampleBuffer: sampleBuffer, options: [:])
do {
            try requestHandler.perform([pathRequest])
        } catch {
            print("Unable to perform the request: \(error).")
        }

Code Block 
func trajectoryHandler(request: VNRequest, error: Error?) {
        print("have trajectory observation")
        guard let observations =
                request.results as? [VNTrajectoryObservation] else { return }
        print(observations)
    }