This is almost always due to Windows Driver Signature Enforcement blocking the Flashtool drivers. Restart Windows in advanced startup mode, disable signature enforcement, and reinstall the drivers.
Re-install the Flashmode drivers and ensure you are holding Volume Down properly. Sony Flashtool 0.9.18.6
Wait until a pop-up window appears on Flashtool instructing you to connect your phone. Turn your Xperia device . Press and hold the Volume Down button. This is almost always due to Windows Driver
Many Sony bootloaders can only be unlocked on older Android versions. Use Flashtool to flash Android 4.0.4 (Ice Cream Sandwich) on an Xperia S, unlock bootloader using the official Sony code, then flash a custom ROM. disable signature enforcement