Clip+ problem with usb (rockbox 3.11)

[france996]

Hello to everyone.
I'm  new to this forum. i've recently bought a sansa clip+ and i've installed on it rockbox 3.11, the latest one. the only problem i have is about the usb connection on my pc. the original firmware works well but when i plug the usb with rockbox nothing happens.
i've put in the original firmware the usb mode to mtp. i own original windows 7 64 bit.
any idea?
thanks a lot!