Make sure you have enough storage space left on your device to download Covet Fashion (it's around 1.5GB) and that you're connected to WiFi or 3G/4G if necessary. Check that your system time and date are set correctly (you can check yours by going to System Settings- >Date & Time) and that you're not trying to download over WiFi when your store's network is down. If all else fails, contact your store; it could be a bug specific to certain devices so they should be able to help you find a solution.