I had the same problem, I used the original clamps that came on the bike. What I did was remove the tapered spacer on the clamp and then tightened it back up. Do not over do it since the spacer is supposed to keep the pipe round. The reason I didn't replace the spacer is here is Korea it isn't worth the cost of having one mailed over. When I need some other parts I will order a new clamp until then this will do.
I put a piece of sheet metal between the clamp and the muffler to compensate for the stretch when i installed my mufflers 50k miles ago...never had a problem again.
I had that problem on my EVO Road king. I don't know if the new clamps are anything like the older ones are but I took the clamps off and modified them. They didn't look like they were made correctly. The metal straps didn't form around the "H" spacer correctly which didn't let the clamps get tight enough. A little work in the vise and now they tighten up just fine. No more loose mufflers. I figured if I bought another set of clamps I would have the same problem.
Cool, thanks everone. Sounds like I need to look at the clamps. I'll get some new ones and torque properly, see if that does it.
You need to call/email Vance and Hines.
I have a full set of dresser duals and hi-output slip ons. When they were delivered, they came with the wrong hardware, both for the clamps and the mounting brackets that attach the SO's to the saddlebag supports.
V&H replaced everything that was incorrect, and I've not had a single issue since.