Creating bootable usb drive with Sierra
Try formatting the USB stick twice as HFS+ (journaled)/GPT. Then use DiskMaker X to write the image. This is a piece of software that is updated very often and usually doesn't have any bugs (at least to my knowledge), so it might work better than the app you are using now. If nothing helps, then you probably have a broken USB stick - go buy something like a SanDisk Ultra USB 3.0 (I have it and it works great).