This started to happen to my C too, somtimes its even hard to click the belt on, i have to either push hard or press the release buton a few times, i will take it to the dealer this Sunday.
My 300 is a 2005. For the first time ever my drivers side seatbelt wont unclick. I somehow wiggled out of it when I got home (not sure what the neighbors thought I was doing!). I have tried blowing on it with the aircompressor and yelling at it. Neither one worked.