Rewardcharts...how many use this option for their children, not working for me

I have a 9 year old boy and we have made a reward chart ,so he can help us around the house ,we have had this for a year or so . Well its not working, he dosnt do all his chores anymore and rewards are not working . I love watching supernanny but my kids are not that bad , i was a preschool teacher for 10 years but not sure how to raise a tween if you want to call him that lol. anyways any ideas how to get him excited for the chore chart? Lost for ideas