It's usually just a glitch. Remove your iPad, plug it back in and try again. You can also try resetting your iPad, hold down the sleep and home keys, past when you see the red power down slider and until you see the silver apple. Let it reboot and see what it does. Also just close and then reopen iTUnes. ANy combo might help.
Sometimes I have to turn my device on, then start the sync if the device is asleep.
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