i had the same thing happen to me and i had to wrap the part of my bicycle where i put the tensioner clamp on with a piece of rubber. then put the clamp on that. i just used the rubber that you put around the rim of tire and it worked. i think the triangle arm on the bike was not big enough to get a tight hold. hope that helps
It doesn't hurt to measure the distance from the back of the sprocket to the rear spoke at each location to make sure they are the same. I did this once the bike shop told me the sprocket was warped. I think I just had it bolted up wrong.