Just a quick tip should anyone come across it (although its rare). Due to the coding of BLheli suite, it cant recognise your ESC if your ESC (or FC in passthrough mode) COM port is above 255.
Over time each time you connect a new serial device to your PC, it will assign a new COM port to that device. Over time this number will sequentially increase, and if you connect loads of devices like us, this number will eventually go above 255.
If this is the case all you need to do is open up device manager and manually change the number to something less than 255. To change this you go to the device properties, Port Settings tab, and click on Advanced. Within that screen you can set the COM port number manually as shown above.