I have the same problem with the muffler nipples on a Saito 180.
Tried loctite, still a problem.
Tried JB weld ... still a problem.
Got desperate and tried Alumiweld ... ruined the muffler ( my fault ).

On my mufflers (have three .. one completely ruined from experimentation) the threaded hole(s) miss the thick aluminum used in the exit port so that only about 3/4 of the hole has threads. Pressure nipple keeps coming loose.

Hanger 9 did send a new muffler after I explained the problem.
Same hole problen ....
Nipple comes loose on it also.

I'm OK with the muffler and exhaust manifold !

Maybe someone else is having the problem and has a fix.

Just to let you know someone else is experiencing some of the same problems.