So, if replacing the clamp does not work, I have a few questions:
On the muffler that is loose:
I am assuming there there are some "cuts" in at least 2 places on the muffler where is slides on the pipe. The "cuts" I am questioning are to allow the muffler flange to squeeze together to make a snug fit on the exhaust pipe. Are these "cuts" touching edges or is there space between them?
Does the space look the same as the muffler that fits well?
My thinking is that if replacing the clamp does not work, then perhaps you have a poorly manufactured muffler that may need to go back, or you could make the cuts a little wider yourself so there is room to compress the muffler.
I had problems removing my left side stock muffler when I first installed my slip ons. The exhause pipe was actually flared slightly wider at the end. My thinking was that when it was installed at the factory it was a real snug fit, or they had to pry open the muffle a bit to get it on. Then they really tightened the torca clamp...but not right at the end of the exhaust pipe. So I had a compressed muffler flange and a slightly expanded exhaust pipe. Really took a beating to get it off.
The use of high temp silicone would concern me if it makes the muffler difficult to remove later. I use a small dab of anti-seize on the fittings to make it easy to remove the muffller for servicing and tire replacement. I don't put tires on the rims myself, but I don't want some tech really putting stress on the exhaust system trying to remove a stubborn muffler to get at the tire/axel.