Are you talking about the rubber hangers that suspend the pipes and silencers to the body, or the actual 'U' clamp that connects the pipes to the silencer?
If its the latter, it can be a good move to increase the width of the slot thats cut into the larger piece......sometimes the clamps close them up and they cant be squeezed more after that.
John
Adam, sometimes it helps to use two clamps, top to bottom and evenly tightened. Exhaust cement also has a bonus of helping to stick the pipes together.
John