OpenCV is not detecting rectangle(paper) In some Devices like moto,oppo devices

I have integrated latest version of opencv(4.1.0) for rectangle(Paper) detection but it is unable to detect rectangle(paper) in some devices like moto,oppo devices. also it is only detecting rectangle with mostly dark background like black , so please help me to find out what should be the problem ? i have used this demo : https://github.com/KePeng1019/SmartPaperScan