My iPod keep saying not enough storage even though it still has more than half left why?

it keeps poping up a notification that says "not enough storage to back up your iPod you can manage your storage settings in the settings" but I looked on it and it says my iPod can has 12.7 gb in all and 6.4 avalible and I don't get it how can I not have enough storage when there is still half my storage left I even deleted most of my photos videos and apps that I didn't need ect it is super annoying and I want it to stop cause it makes no sense please help.