Are you connecting the USB flash drive to a USB 3 port (most likely this will have a blue plastic insert)? If so, use a USB 2 port instead (likely with a black plastic insert). If your laptop only has USB 3 ports, you'll have to slipstream the appropriate USB 3 driver into your installation ISO file because Windows 7 doesn't include USB 3 drivers in the installation media.
See, for one out of many online examples, https://community.spiceworks.com/how_to/125921-how-to-add-drivers-manually-to-usb-drive-to-install-windows-using-a-usb-3-0-port