TR,

Sometimes the muffler bolts are a hair long, and are bottoming out in the holes before they are really tight. You might want to check that. If so, just shorten them a bit at a time until they no longer bottom out when you tighten them.

Another way to help keep them tight is to insert a piece of monofilament line into the bolt hole and then insert the bolt. It acts just like the nylon insert in a nut.