It's a hardware bug on the nano board.
At least, it always has been for me. Is it more reliable if you just plug the nano in without any circuitry connected to the nano?
The nano uses a DS_FT232RL for usb (chip on the bottom). If you look at the data sheet http://www.ftdichip.com/Support/Documen ... FT232R.pdf
you will find that pin 26 is 'Test' - "Puts the device into IC test mode. Must be tied to GND for normal operation, otherwise the device will appear to fail." I expect you might see where this is heading already...
If you look at the nano schematic http://arduino.cc/en/uploads/Main/Ardui ... ematic.pdf
pin 26 on the FT232RL is ... disconnected.
However, you will notice that pin 25 is connected to ground on the nano. So a solder bridge between pin 25 and pin 26 will solve your problems. At least, it has solved all of mine (ok, maybe not all, all the ones with USB on an arduino nano though).
You want a solder blob between pins 25 and 26, being the fourth and third down on the right hand side when you look at the chip with the dot at top left. Obviously, you don't want to short the adjacent pins into this blob. Good luck. Alternatively, if you look further up the board you can see a reasonable expanse of ground plane in the traces, and notice that the top end of the right SMD component just above the chip is connected to that. You could put a flying wire from pin 26 to the top end of that component. I think the solder bridge between legs is easier.
I'm assuming you have a V3 nano. That's all I've ever used.
Edit: I believe Gravitech have fixed their own 'genuine' boards. I believe there are an awful lot of half-the-price chinese duplicates that aren't fixed. I've only ever bought cheap clones, and the boards I have all show the same problem.