My mac doesn't recognize my ipod nano

i have a 2nd gen ipod nano and it works perfectly fine when plugged into other lap tops and computers but recently it isnt recognized on my mac computer. i plug it in through the usb port and itunes or the rest of the computer doesnt well...see it? it doesnt come up with any message saying its not working, my ipod just doesnt come up as being plugged into the mac but yet its charging while being plugged into the computer? Ive tride plugging it into different usb ports on the mac, using different cables, but nothing works! all it does is charge my ipod. my usb ports on my computer work fine with other things plugged into it so thats not broken...help please!