First if you havnt done this already, I would recommend
Turning iPod nano off completely and turning it on again
Hold down the Sleep/Wake button for several seconds to turn iPod nano off completely.
Note: If the iPod nano (6th generation) is showing that it's charging or attached to a powered accessory or cable, you cannot turn off the iPod until you detach the accessory or cable from the device.
To turn iPod nano back on, hold down the Sleep/Wake button for a few seconds.
From: iPod nano (6th generation): Software update version 1.1 changes
And if that doesn not quite cut the mustard, I would recommend restoring the device in iTunes.
Restoring iPod to factory settings